#  JUNO Physics and Detector

JUNO Collaboration [1]

###### Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid
scintillator detector in a laboratory at 700-m underground. An excellent
energy resolution and a large fiducial volume offer exciting opportunities for
addressing many important topics in neutrino and astro-particle physics. With
six years of data, the neutrino mass ordering can be determined at a
3-4$\sigma$ significance and the neutrino oscillation parameters
$\sin^{2}\theta_{12}$, $\Delta m^{2}_{21}$, and $|\Delta m^{2}_{32}|$ can be
measured to a precision of 0.6% or better, by detecting reactor antineutrinos
from the Taishan and Yangjiang nuclear power plants. With ten years of data,
neutrinos from all past core-collapse supernovae could be observed at a
3$\sigma$ significance; a lower limit of the proton lifetime, $8.34\times
10^{33}$ years (90% C.L.), can be set by searching for $p\to\bar{\nu}K^{+}$;
detection of solar neutrinos would shed new light on the solar metallicity
problem and examine the vacuum-matter transition region. A typical core-
collapse supernova at a distance of 10 kpc would lead to $\sim 5000$ inverse-
beta-decay events and $\sim 2000$ (300) all-flavor neutrino-proton (electron)
elastic scattering events in JUNO. Geo-neutrinos can be detected with a rate
of $\sim 400$ events per year. Construction of the detector is very
challenging. In this review, we summarize the final design of the JUNO
detector and the key R&D achievements, following the Conceptual Design Report
in 2015 [2]. All 20-inch PMTs have been procured and tested. The average
photon detection efficiency is 28.9% for the 15,000 MCP PMTs and 28.1% for the
5,000 dynode PMTs, higher than the JUNO requirement of 27%. Together with the
$>20$ m attenuation length of the liquid scintillator achieved in a 20-ton
pilot purification test and the $>96\%$ transparency of the acrylic panel, we
expect a yield of 1345 photoelectrons per MeV and an effective relative energy
resolution of $3.02\%/\sqrt{E{\rm(MeV)}}$ in simulations [3]. To maintain the
high performance, the underwater electronics is designed to have a loss rate
$<0.5\%$ in six years. With degassing membranes and a micro-bubble system, the
radon concentration in the 35 kton water pool could be lowered to $<10$
mBq/m3. Acrylic panels of radiopurity $<0.5$ ppt U/Th for the 35.4-m diameter
liquid scintillator vessel are produced with a dedicated production line. The
20 kton liquid scintillator will be purified onsite with Alumina filtration,
distillation, water extraction, and gas stripping. Together with other low
background handling, singles in the fiducial volume can be controlled to $\sim
10$ Hz. The JUNO experiment also features a double calorimeter system with
25,600 3-inch PMTs, a liquid scintillator testing facility OSIRIS, and a near
detector TAO.

###### Contents

  1. 1 Introduction
    1. 1.1 Overview of the JUNO experiment
    2. 1.2 JUNO experimental site
    3. 1.3 JUNO detector
    4. 1.4 JUNO signal and background
  2. 2 Physics with JUNO
    1. 2.1 Neutrino mass ordering
    2. 2.2 Precision measurement of neutrino oscillation parameters
    3. 2.3 Supernova neutrinos and trigger strategy
    4. 2.4 Diffuse supernova neutrino background
      1. 2.4.1 DSNB signal
      2. 2.4.2 Background rejection
      3. 2.4.3 DSNB sensitivity
    5. 2.5 Solar neutrinos
      1. 2.5.1 Measurement of 8B solar neutrinos
      2. 2.5.2 Potential of measuring $pp$ and 7Be neutrinos
    6. 2.6 Atmospheric neutrinos
    7. 2.7 Geoneutrinos
      1. 2.7.1 Potential of geoneutrino measurement in JUNO
      2. 2.7.2 Refined crustal models and expected signal for JUNO
    8. 2.8 Nucleon decays
    9. 2.9 Other physics
  3. 3 Detector design and R&D
    1. 3.1 Detector design overview
    2. 3.2 Central Detector
      1. 3.2.1 Central Detector scheme and requirements
      2. 3.2.2 The LS container: Acrylic Vessel
      3. 3.2.3 Main Structure
      4. 3.2.4 The Filling, Overflow, and Circulation (FOC) system
    3. 3.3 Veto detector
      1. 3.3.1 Water Cherenkov Detector (WCD)
      2. 3.3.2 Top Tracker (TT)
    4. 3.4 Liquid scintillator
      1. 3.4.1 Purification systems
      2. 3.4.2 OSIRIS
    5. 3.5 20-inch PMT Test and Instrumentation
      1. 3.5.1 20-inch PMT Test
      2. 3.5.2 20-inch PMT Instrumentation
    6. 3.6 The large PMT readout and trigger electronics
    7. 3.7 Small PMT system
      1. 3.7.1 Overview and motivation
      2. 3.7.2 3-inch photomultipliers production and instrumentation
      3. 3.7.3 Underwater front-end electronics
    8. 3.8 Calibration
      1. 3.8.1 Calibration system design
      2. 3.8.2 Calibration sources and program
      3. 3.8.3 Auxiliary systems
    9. 3.9 Data acquisition and detector control
      1. 3.9.1 Data Acquisition System (DAQ)
      2. 3.9.2 Detector Control System (DCS)
    10. 3.10 Offline software and computing
      1. 3.10.1 Framework
      2. 3.10.2 Simulation
      3. 3.10.3 Reconstruction
      4. 3.10.4 Performance of Simulation and Reconstruction
      5. 3.10.5 Database and DQM
      6. 3.10.6 Computing
    11. 3.11 Low background control
    12. 3.12 TAO
  4. 4 Summary

##  1 Introduction

###  1.1 Overview of the JUNO experiment

The standard three-flavor neutrino oscillation pattern is well established
after the observation of the neutrino oscillation in solar, atmospheric,
accelerator and reactor neutrino experiments. Two independent neutrino mass
splittings $|\Delta m^{2}_{31}|=|m^{2}_{3}-m^{2}_{1}|$ (or $|\Delta
m^{2}_{32}|=|m^{2}_{3}-m^{2}_{2}|$) and $\Delta
m^{2}_{21}=m^{2}_{2}-m^{2}_{1}$, and three neutrino mixing angles from the
Pontecorvo-Maki-Nakagawa-Sakata (PMNS) parametrization [4, 5] were measured
with precisions at a level of few percents. However, several unknowns still
exist and will be the focus of future neutrino oscillation experiments. They
include

  * •

the Neutrino Mass Ordering (NMO),

  * •

the leptonic CP-violating phase $\delta$ in the PMNS matrix,

  * •

the octant of the mixing angle $\theta_{23}$ (i.e., $\theta_{23}<\pi/4$ or
$\theta_{23}>\pi/4$).

The Jiangmen Underground Neutrino Observatory (JUNO), a 20 kton multi-purpose
underground liquid scintillator detector, was proposed with the determination
of the neutrino mass ordering as a primary physics goal [6, 7, 8, 9]. The
neutrino mass ordering has only two possibilities: the normal ordering (NO,
$m_{1}<m_{3}$) and the inverted ordering (IO, $m_{1}>m_{3}$). The relatively
large value of $\theta_{13}$ has provided excellent opportunities to resolve
the NMO in various neutrino oscillation experiments, which include a medium
baseline ($\sim$50 km) reactor antineutrino $\bar{\nu}_{e}\to\bar{\nu}_{e}$
oscillation experiment (JUNO), long-baseline accelerator (anti-)neutrino
$\nu_{\mu}\to\nu_{e}$ oscillation experiments (NO$\nu$A [10] and DUNE [11]),
and atmospheric (anti-)neutrino oscillation experiments (INO [12], PINGU [13],
ORCA [14], DUNE [11] and Hyper-K [15]). The accelerator and atmospheric
experiments rely on the matter effect in neutrino oscillations (the charge-
current interaction between (anti-)$\nu_{e}$ and electrons in the matter).
JUNO is a unique experiment designed to identify the NMO using the oscillation
interplay between $\Delta m^{2}_{31}$ and $\Delta m^{2}_{32}$ [16]. The NMO
sensitivity of JUNO has no dependence on the unknown CP-violating phase and
the $\theta_{23}$ octant, playing a key role when combined with other neutrino
experiments.

The reactor antineutrino survival probability in vacuum can be written as

$\displaystyle
P_{\bar{\nu}_{e}\to\bar{\nu}_{e}}=1-\sin^{2}2\theta_{13}(\cos^{2}\theta_{12}\sin^{2}\Delta_{31}+\sin^{2}\theta_{12}\sin^{2}\Delta_{32})-\cos^{4}\theta_{13}\sin^{2}2\theta_{12}\sin^{2}\Delta_{21},$
(1)

where $\Delta_{ij}=\Delta m_{ij}^{2}L/(4E)=(m_{i}^{2}-m_{j}^{2})L/(4E)$, in
which L is the baseline and E is the antineutrino energy. At a baseline of 53
km, JUNO will simultaneously measure oscillations driven by small mass
splitting ($\Delta m^{2}_{21}$) and large mass splitting ($\Delta m^{2}_{31}$
and $\Delta m^{2}_{32}$) as shown in Fig. 1.

Figure 1:  The expected antineutrino energy spectrum weighted by IBD cross-
section with and without oscillation at the JUNO experiment for normal
ordering and inverted ordering assuming 2000 days of data-taking. Dependence
on the four oscillation parameters is shown.

The small oscillation peaks in the oscillated antineutrino spectrum contain
the NMO information. Precise measurement of the oscillated antineutrino
spectrum is a key for JUNO to determine the NMO. This requires a 20 kton
liquid scintillator detector with an unprecedented relative energy resolution
of $\sigma_{\rm E}/E=3\%/\sqrt{E_{\mathrm{vis}}}$, with $E_{\mathrm{vis}}$
being the visible energy in the detector in MeV.

Besides the neutrino mass ordering, the large fiducial volume and the
excellent energy resolution of JUNO offer exciting opportunities for
addressing many important topics in neutrino and astro-particle physics.

The precision measurement of reactor antineutrino spectrum will also lead to
the precise determination of the neutrino oscillation parameters
$\sin^{2}\theta_{12}$, $\Delta m^{2}_{21}$, and $|\Delta m^{2}_{32}|$ as
illustrated in Fig. 1. The expected accuracy of these measurements will be
better than 0.6%, which will play a crucial role in the future unitarity test
of the PMNS matrix.

The JUNO detector is not limited to detect antineutrinos from the reactors,
but also observe neutrinos/antineutrinos from terrestrial and extra-
terrestrial sources, including supernova burst neutrinos, diffuse supernova
neutrino background, geoneutrinos, atmospheric neutrinos, and solar neutrinos.
For example, a neutrino burst from a typical core-collapse supernova at a
distance of 10 kpc (kiloparsec ) would lead to $\sim 5000$ inverse-beta-decay
events and $\sim 2000$ all-flavor neutrino-proton elastic scattering events in
JUNO, which are of crucial importance for understanding the mechanism of
supernova explosion and for exploring novel phenomena such as collective
neutrino oscillations. Detection of 1–2 neutrinos per year from all past core-
collapse supernova explosions in the visible universe can further provide
valuable information on the cosmic star-formation rate and the average core-
collapse neutrino energy spectrum. Antineutrinos originating from the
radioactive decay of uranium and thorium in the Earth can be detected in JUNO
with a rate of $\sim 400$ events per year, significantly improving the
statistics of existing geoneutrino event samples. Atmospheric neutrino events
collected in JUNO can provide independent inputs for determining the mass
ordering and the octant of the $\theta_{23}$ mixing angle. Detection of the
7Be and 8B solar neutrino events at JUNO would shed new light on the solar
metallicity problem and examine the spectral transition region between the
vacuum and matter-dominated neutrino oscillations.

The JUNO detector provides sensitivity to physics searches beyond the Standard
Model. As examples, we highlight the searches for proton decay via the $p\to
K^{+}+\bar{\nu}$ decay channel, neutrinos resulting from dark-matter
annihilation in the Sun, violation of Lorentz invariance via the sidereal
modulation of the reactor neutrino event rate, and the effects of non-standard
neutrino interactions.

JUNO was first conceived in 2008 [6, 7]. It was approved in 2013 after Daya
Bay [17], Double Chooz [18], and RENO [19] measured an unexpectedly large
value of $\theta_{13}$, which meant that the NMO could be determined with
current technologies. The civil construction started in 2015. The detector is
expected to be ready in 2022, and data-taking is expected in 2023.

In 2018, the Taishan Antineutrino Observatory (TAO, also known as JUNO-TAO)
was proposed as a satellite experiment of JUNO to measure the reactor
antineutrino spectrum with sub-percent energy resolution [20]. Since Daya Bay
[21], Double Chooz [22], and RENO [23], among others, have found that the
model prediction on the reactor antineutrino spectrum [24, 25] has large
discrepancies with data, TAO will provide a reference spectrum for JUNO, and
also provide a benchmark measurement to test nuclear databases. TAO will be a
ton-level liquid scintillator detector at $\sim 30$ meters from a reactor core
of the Taishan Nuclear Power Plant (NPP). It is expected to start operation at
a similar time scale as JUNO.

The physics potential of the JUNO experiment has been explored in the JUNO
Yellow Book [9]. In this article, we will review the major goals with updated
inputs and improved analyses. Since the release of the Conceptual Design
Report of JUNO [2], the detector design has been further optimized. Challenges
regarding the detector technologies have been solved with extensive R&D. The
final design of the JUNO detector and key R&D achievements will be summarized
in this article.

###  1.2 JUNO experimental site

The JUNO experiment is located in Jinji town, 43 km to the southwest of
Kaiping city, a county-level city in the prefecture-level city Jiangmen in
Guangdong province, China. The geographic location is 112∘31’05” E and
22∘07’05” N. The distances to several megacities, Guangzhou, Shenzhen, and
Hong Kong, are all around 200 km.

As shown in Fig. 2, the experimental site is at equal distances of $\sim$ 53
km from the Yangjiang NPP and the Taishan NPP, optimized to have the best
sensitivity for determining the mass ordering.

Figure 2: JUNO location.

Reactor antineutrino is the primary neutrino source in the JUNO detector.
Yangjiang NPP has six reactor cores of 2.9 GWth each (thermal power). All
cores are the second-generation pressurized water reactors CPR1000, which is a
derivative of Framatome M310. The distances between any two cores of the
Yangjiang NPP are between 89 m and 736 m. All six cores are in operation.
Taishan NPP has two cores of 4.6 GWth each in operation. Both are third-
generation pressurized water reactors EPR. The distance between the two cores
is 252.5 m. Possibly another two cores in the Taishan NPP will be built in the
future, but the plan is unclear for now. The total thermal power of the
Yangjiang and Taishan NPPs is 26.6 GWth.

The Daya Bay nuclear complex is 215 km away from the JUNO detector. It
includes the Daya Bay NPP, the Ling Ao NPP, and the Ling Ao-II NPP in a spread
of 1.1 km, each with 2 cores of 2.9 GWth. The Daya Bay and Ling Ao cores are
Framatome M310 and the Ling Ao-II cores are CPR1000. They will contribute
about 6.4% of the reactor antineutrino events in the JUNO detector considering
oscillation. Huizhou NPP is under construction with six 2.9 GWth reactor cores
and is expected to be ready around 2025. The plan of Lufeng NPP is unclear
now. The Huizhou site is 265 km away from the JUNO detector and the Lufeng
site is more than 300 km away. There is no other NPP or planned NPP in a
radius of 500 km around the JUNO experimental site.

The thermal power of all cores and the baselines (distances to the JUNO
detector) are listed in Tab. 1. The distances from the detector site to the
Yangjiang and Taishan cores are surveyed with a Global Positioning System
(GPS) to a precision of 1 meter. All these NPPs are constructed and operated
by the China General Nuclear Power Group (CGNPG).

Cores | YJ-1 | YJ-2 | YJ-3 | YJ-4 | YJ-5 | YJ-6 | TS-1 | TS-2 | DYB | HZ
---|---|---|---|---|---|---|---|---|---|---
Power (GW) | 2.9 | 2.9 | 2.9 | 2.9 | 2.9 | 2.9 | 4.6 | 4.6 | 17.4 | 17.4
Baseline(km) | 52.74 | 52.82 | 52.41 | 52.49 | 52.11 | 52.19 | 52.77 | 52.64 | 215 | 265
Table 1: Summary of the thermal power and baseline to the JUNO detector for
the Yangjiang (YJ) and Taishan (TS) reactor cores, as well as the remote
reactors of Daya Bay (DYB) and Huizhou (HZ).

Due to the absence of high mountains in the allowed area where the sensitivity
to the mass ordering is optimized, the JUNO detector will be deployed in an
underground laboratory under the Dashi hill. Currently, the experiment hall
has been excavated. The location has been shifted by $\sim$60 m to the
northwest of the originally designed location in Ref. [9]. The elevation of
the hill above the detector is 240.6 m above sea level. The dome and the floor
of the underground experimental hall are at -403.5 m and -430.5 m,
respectively. The detector is located in a cylindrical pit, with the detector
center at -452.75 m. Therefore, the vertical overburden for the detector
center is 693.35 m (1800 m.w.e). The experimental hall will have two accesses:
a 564 m deep vertical shaft and a 1266 m long tunnel with a slope of 42.5%.
The surrounding rock is granite. The average rock density along a 650 m
borehole near the experimental hall is measured to be 2.61 g/cm3. The
activities of the 238U, 232Th, and 40K in the rock around the experimental
hall are measured to be 120, 106, and 1320 Bq/kg, respectively, with 10%
uncertainties. The muon rate and average energy in the JUNO detector are 0.004
Hz/m2 and 207 GeV estimated by simulation, taking the surveyed mountain
profile into account.

###  1.3 JUNO detector

The JUNO detector consists of a Central Detector (CD), a water Cherenkov
detector and a Top Tracker (TT). A schematic view of the JUNO detector is
shown in Fig. 3. The CD is a liquid scintillator (LS) detector with a designed
effective energy resolution of $\sigma_{\rm E}/E=3\%/\sqrt{E{\rm(MeV)}}$. It
contains 20 kton LS in a spherical acrylic vessel, which is submerged in a
water pool. The acrylic vessel is supported by a stainless steel (SS)
structure via Connecting Bars. The CD Photomultiplier Tubes (PMTs) are
installed on the inner surface of the SS structure. The water pool is equipped
with PMTs to detect the Cherenkov light from cosmic muons, acting as a veto
detector. Compensation coils are mounted on the SS structure to suppress the
Earth’s magnetic field and minimize its impact on the photoelectron collection
efficiency of the PMTs. The CD and the water Cherenkov detector are optically
separated. On top of the water pool, there is a plastic scintillator array,
i.e. Top Tracker, to accurately measure the muon tracks. A chimney for
calibration operations connects the CD to the outside from the top. The
calibration systems are operated in the Calibration House, above which a
special radioactivity shielding and a muon detector are designed. Detailed
description of the detector can be found in Sec. 3.

Figure 3: Schematic view of the JUNO detector

###  1.4 JUNO signal and background

Reactor antineutrinos constitute the primary signal in the JUNO detector for
determining the neutrino mass ordering and for precision measurements of the
neutrino oscillation parameters. All reactors close to JUNO are commercial
light-water reactors, where fissions of four isotopes in fuel, 235U, 238U,
239Pu and 241Pu, contribute $>99.7\%$ of the antineutrinos. The antineutrino
flux is composed of these four components weighted by the fission rate of the
four isotopes, $\phi(E_{\nu})=\sum_{i}^{4}F_{i}S_{i}(E_{\nu})$, where $F_{i}$
is the fission rate and $S_{i}(E_{\nu})$ is the antineutrino energy spectrum
per fission for the $i$-th isotope. The fission rate can be evaluated based on
the reactor running information provided by the NPPs, including the reactor
thermal power, the burn-up of the fuel, the fission fractions of four
isotopes, and the energy released per fission. The antineutrino spectrum per
fission has been calculated using two methods. One is based on the summation
method [26, 27, 28] which sums all the antineutrino energy spectra
corresponding to thousands of beta decay branches for about 1000 isotopes in
the fission products, utilizing information in nuclear databases. The other is
the beta conversion method [24, 25, 29, 30, 31, 32] which converts the
measured $\beta$ energy spectra from the individual fission isotopes 235U,
239Pu, and 241Pu to the corresponding antineutrino energy spectra. The 238U
spectrum relies on the summation method and contributes $<10$% of the total
events. Recent findings of the reactor antineutrino flux and spectrum
anomalies have revealed unclear systematic effects in the reactor flux models.
To provide a reliable reference antineutrino spectrum, the JUNO-TAO experiment
was proposed as a satellite experiment of JUNO to measure the reactor
antineutrino spectrum with sub-percent energy resolution [20].

JUNO detects electron antineutrinos via inverse beta decay (IBD) interactions,
$\bar{\nu}_{e}+p\to e^{+}+n$. The $e^{+}$ quickly deposits its energy and
annihilates into two 0.511-MeV photons, which provides a prompt signal. The
prompt energy contains both the positron kinetic energy $T_{e^{+}}$ and the
annihilation energy of $2\times 0.511$ MeV. The neutron is mainly captured on
protons. After approximately 200 $\mu s$ of scattering in the detector, the
capture releases a 2.2-MeV photon, providing a delayed signal. A set of
preliminary antineutrino selection cuts is listed below:

  * •

fiducial volume cut $r<17.2$ m;

  * •

the prompt energy cut 0.7 MeV $<E_{p}<$ 12 MeV;

  * •

the delayed energy cut 1.9 MeV $<E_{d}<$ 2.5 MeV;

  * •

time interval between the prompt and delayed signal $\Delta T<1.0$ ms;

  * •

the prompt-delayed distance cut $R_{p-d}<1.5$ m;

  * •

muon veto criteria:

    * –

for muons tagged by the water Cerenkov detector or the Top Tracker, veto the
whole LS volume for 1.5 ms;

    * –

for well-tracked muons in the Central Detector, veto the detector volume
within a cylinder of distance to the muon track $R_{d2\mu}<3$ m and within
time to the preceding muon $T_{d2\mu}<1.2$ s;

    * –

for tagged, non-trackable muons in the Central Detector, veto the whole LS
volume for 1.2 s.

The detection efficiency for each cut is shown in Tab. 3. JUNO will detect 60
IBDs/day with the above selection criteria.

The two major background sources of JUNO are natural radioactivity and the
products of cosmic muons. Natural radioactivity comes from all materials and
the environment. Huge efforts on material screening and a careful arrangement
of the experimental apparatus reduce the single event rate to about 10 Hz in
the fiducial volume. The accidental background due to radioactivity and
neutrons is further reduced by applying selection cuts using the energy, time
and space signatures of both prompt and delayed signals. Moreover,
13C($\alpha$,n)16O reactions in the liquid scintillator result in correlated
background events. For the reactor antineutrino program, the U/Th
concentration is required to be lower than $10^{-15}$ g/g in the liquid
scintillator, while the requirement is $10^{-17}$ g/g for the solar neutrino
detection. With 693.35 m (1800 m.w.e) vertical overburden, the muon flux in
the detector is 0.004 Hz/m2. The double muon veto systems, formed by a Top
Tracker system and a water Cherenkov detector, ensure a high muon tagging
efficiency to reject the cosmogenic backgrounds: 9Li/8He and fast neutron
backgrounds. An optimized veto strategy has been developed to obtain a $<3\%$
background to signal ratio for 9Li/8He and fast neutron backgrounds. The veto
strategy utilizes the correlation time and distance to the parent muon, the
muon track and energy deposition, the pulse shapes and the energies of prompt-
delayed event pairs. Geoneutrinos, produced by U/Th in the Earth, have the
same signatures as the reactor antineutrinos. Features of the energy spectrum
shapes in the region of $<3$ MeV help to separate the geoneutrino components
with a fraction of $\sim 2\%$. The ratio of the residual backgrounds to the
IBD signal, including geoneutrinos, accidentals, 9Li/8He, fast neutrons and
13C($\alpha$,n)16O is estimated to be about 6% using the same method as Ref.
[9].

The JUNO detector requires an energy non-linearity uncertainty of better than
1% and a $3\%/\sqrt{E}$ effective energy resolution to determine the neutrino
mass ordering. The relation between the true energy and the detected energy is
non-linear due to quenching effects and Cherenkov light emission. The PMT
instrumentation and readout electronics may contribute additional non-
linearity for each channel. Various calibration sources covering most of the
IBD energy range will be deployed regularly to calibrate the energy scale to a
sub-percent level [3]. The novel dual calorimetry with 20-inch and 3-inch PMTs
enables a clean determination of the instrumental non-linearity.

As shown in Fig. 1, the NMO sensitivity relies on the difference of the
multiple small oscillation pattern driven by $\Delta m^{2}_{31}$ in normal and
inverted mass ordering cases. Precise measurement of the oscillation pattern
driven by $\Delta m^{2}_{31}$ requires unprecedented effective energy
resolution. Otherwise, the small oscillation pattern will be washed out. The
unprecedented energy resolution puts stringent requirements on the
transparency of the scintillator and the detection efficiency of PMTs. With
the R&D achievements described in this article, a yield of 1345 p.e./MeV at
the detector center is obtained in simulations based on the nominal detector
parameters. This contributes a statistical term of 2.73% in the energy
resolution. The p.e. yield in the detector is position-dependent. A multi-
positional source deployment calibration strategy is devolved to correct the
non-uniformity. In general, the fractional energy resolution for a visible
energy $E_{\rm vis}$ can be written as an approximate formula

$\frac{\sigma_{E_{\rm vis}}}{E_{\rm vis}}=\sqrt{\left(\frac{a}{\sqrt{E_{\rm
vis}}}\right)^{2}+b^{2}+\left(\frac{c}{E_{\rm vis}}\right)^{2}}\,,$ (2)

where the $a$ term is the statistical term driven by photostatistics, the $b$
term is dominated by the position non-uniformity, and the $c$ term represents
the contribution of background noises. For the neutrino mass ordering
determination, it was found that the impact of the $b$ term is 1.6 times
larger than that of the $a$ term, and the impact of the $c$ term is 1.6 times
smaller than that of the $a$ term [9]. Therefore, an effective energy
resolution can be defined as $\sigma_{\rm eff}/E=a_{\rm
eff}/\sqrt{E{\rm(MeV)}}$, with

$a_{\rm eff}\equiv\sqrt{a^{2}+(1.6\times
b)^{2}+\left(\frac{c}{1.6}\right)^{2}}\,.$ (3)

Detailed studies on the effective energy resolution that could be achieved
with the JUNO detector calibration strategy can be found in Ref. [3], taking
into account the non-uniformity, vertex smearing, PMT quantum efficiency
variation and charge resolution, energy nonlinearity, and PMT noises, etc. For
the nominal setup, the $a$ term is 2.61% (the 2.73% mentioned above
corresponds to the statistics at the detector center while the p.e. yield
increases with radius of the vertex), the $b$ term is 0.82%, and the $c$ term
is 1.23%. Thus, an effective energy resolution of $3.02\%/\sqrt{E{\rm(MeV)}}$
is expected for the JUNO MO determination in simulations.

As a multiple-purpose detector, JUNO is sensitive to a range of neutrino
sources beyond the reactor antineutrinos. The expected signal and background
estimates for various researches and the corresponding physics potentials will
be reviewed in Section 2. The expected neutrino signal rates and major
background sources are summarized in Tab. 2. Only the MO determination
requires a 3% energy resolution. The $|\Delta m^{2}_{32}|$ measurement
benefits moderately from a high energy resolution. All other studies are not
sensitive to the energy resolution. Solar neutrino studies require a U/Th
radiopurity of the LS of $1\times 10^{-17}$ g/g. Reactor and Geoneutrino
studies require $1\times 10^{-15}$ g/g and other studies are not sensitive.

Research | Expected signal | Energy region | Major backgrounds
---|---|---|---
Reactor antineutrino | 60 IBDs/day | 0–12 MeV | Radioactivity, cosmic muon
Supernova burst | 5000 IBDs at 10 kpc | 0–80 MeV | Negligible
| 2300 elastic scattering |  |
DSNB (w/o PSD) | 2–4 IBDs/year | 10–40 MeV | Atmospheric $\nu$
Solar neutrino | hundreds per year for 8B | 0–16 MeV | Radioactivity
Atmospheric neutrino | hundreds per year | 0.1–100 GeV | Negligible
Geoneutrino |  $\sim 400$ per year | 0–3 MeV | Reactor $\nu$
Table 2: Summary of detectable neutrino signals in the JUNO experiment and the
expected signal rates and major background sources.

##  2 Physics with JUNO

###  2.1 Neutrino mass ordering

JUNO determines the NMO using the oscillation interplay between $\Delta
m^{2}_{31}$ and $\Delta m^{2}_{32}$ at a medium reactor baseline ($\sim$53
km). The reactor antineutrino survival probability
$P_{\bar{\nu}_{e}\to\bar{\nu}_{e}}$ is shown in Eq. 1. Although the NMO can be
determined as well with long-baseline accelerator or atmospheric neutrino
experiments, JUNO is unique since its sensitivity is based on the vacuum
oscillations while accelerator and atmospheric experiments rely on the NMO
dependence of matter effects. The JUNO NMO sensitivity has no dependence on
the unknown CP-violating phase and the $\theta_{23}$ octant, adding unique
information when combined with other neutrino experiments.

At a baseline of 53 km, JUNO will simultaneously measure oscillations driven
by the small mass splitting ($\Delta m^{2}_{21}$) and the large mass splitting
($\Delta m^{2}_{31}$ and $\Delta m^{2}_{32}$) as shown in Fig. 1. The small
oscillation peaks in the oscillated antineutrino spectrum contain the NMO
information. Precise measurement of the oscillated antineutrino spectrum is a
key for JUNO to determine NMO. This requires a 20 kton liquid scintillator
detector with an unprecedented effective energy resolution of
3%$/\sqrt{E({\mathrm{MeV}})}$.

JUNO detects the reactor antineutrino signal via IBD reaction,
$\bar{\nu}_{e}+p\rightarrow e^{+}+n$. The reactor antineutrino interacts with
a proton, creating a positron ($e^{+}$) and a neutron. The positron quickly
deposits its energy and annihilates into two 0.511-MeV $\gamma$-rays, which
gives a prompt signal. The neutron scatters in the detector until
thermalization. It is then captured on a proton (99%) or carbon (1%) within an
average time of $\sim\,$200 $\mu$s and produces gammas of 2.2 MeV or 4.95 MeV.
The coincidence of the prompt-delayed signal pair in such a short time
significantly reduces backgrounds. The observable neutrino energy spectrum can
be approximately obtained from the prompt signal with a shift of $\sim\,$0.8
MeV.

Accidental coincidence background, 8He/9Li, fast neutrons and 13C($\alpha$,
n)16O reactions are the major backgrounds for the reactor neutrino program in
JUNO. A fiducial volume cut can significantly reduce the accidental and the
13C($\alpha$, n)16O backgrounds. Energy selection, time coincidence, and
vertex correlation of the prompt and delayed signals are used in the
antineutrino selection to further suppress the accidental background. To
reject the cosmogenic backgrounds such as 9Li/8He and fast neutrons, muon veto
cuts need to be optimized to maximize the detector live time and minimize the
dead volume losses. A set of preliminary selection criteria is studied in the
JUNO Yellow Book [9]. Tab. 3 summarizes the efficiencies of the selection cuts
and the corresponding reductions for various backgrounds. JUNO will observe 60
IBD events per day, with about 6% background contamination.

Selection | IBD efficiency | IBD | Geo-$\nu$s | Accidental |  9Li/8He | Fast $n$ | $(\alpha,n)$
---|---|---|---|---|---|---|---
- | - | 83 | 1.5 | - | 84 | - | -
Fiducial volume | 91.8% | 76 | 1.4 |  | 77 | 0.1 | 0.05
Energy cut | 97.8% |  |  | 410 |  |  |
Time cut | 99.1% | 73 | 1.3 |  | 71 |  |
Vertex cut | 98.7% |  |  | 1.1 |  |  |
Muon veto | 83% | 60 | 1.1 | 0.9 | 1.6 |  |
Combined | 73% | 60 | 3.55
Table 3: The efficiencies of antineutrino selection cuts, signal and
backgrounds rates, taken from Ref. [9].

To obtain the NMO sensitivity, we employ the least-squares method and
construct a $\chi^{2}$ function as

$\chi^{2}_{\mathrm{REA}}=\sum^{N_{\mathrm{bin}}}_{i=1}\frac{[M_{i}-T_{i}(1+\sum_{k}\alpha_{ik}\epsilon_{k})]^{2}}{M_{i}}+\sum_{k}\frac{\epsilon^{2}_{k}}{\sigma^{2}_{k}}\,,$
(4)

where $M_{i}$ is the number of measured neutrino events in the $i$-th energy
bin, $T_{i}$ is the predicted number of neutrino events with oscillations,
$\sigma_{k}$ is a systematic uncertainty, $\epsilon_{k}$ is the corresponding
pull parameter, and $\alpha_{ik}$ is the fraction of neutrino event
contribution of the $k$-th pull parameter to the $i$-th energy bin. The
considered systematic uncertainties include the correlated (absolute) reactor
uncertainty ($2\%$), the uncorrelated (relative) reactor uncertainty
($0.8\%$), the background uncertainties, the spectral shape uncertainty
($1\%$) and the non-linearity uncertainty. We fit the spectrum assuming the
normal ordering or inverted ordering with the chi-squared method and take the
difference of the minima as a measure of the median NMO sensitivity. The
discriminator of the NMO can be defined as

$\Delta\chi^{2}_{\mathrm{MO}}=|\chi^{2}_{\rm min}(\rm NO)-\chi^{2}_{\rm
min}(\rm IO)|,$ (5)

where the minimization process is implemented for all the relevant nuisance
parameters including oscillation parameters. With the inputs described above,
we obtain $\Delta\chi^{2}=10$, which shows the NMO can be determined with a
significance of $3\sigma$ in 6 years of data taking [9]. Several analyses
implementing different approaches have yielded consistent results.

Recent results on the measurement of the antineutrino energy spectrum show an
anomaly on the spectral shape, known as the 5–MeV bump. In addition, fine
structure [33] may exist in the antineutrino energy spectrum. These findings
indicate that the uncertainty in the reactor antineutrino flux model may be
underestimated. The Taishan Antineutrino Observatory (TAO) has been proposed
to provide a precise model-independent reference spectrum for JUNO to reduce
the flux uncertainty [20]. The uncertainty of this reference spectrum is
estimated to be $\sim 1\%$ over most of the energy region, taking into account
the statistics with 3 years of data, energy non-linearity effect, energy
leakage in a small detector, and fission fraction uncertainties when
predicting other reactor cores with the measurement of only one core.

Compared to the assumptions in the Yellow Book, several changes have occurred
to the experimental inputs.

  * •

It has become clear that two of the four planned Taishan reactor cores will
not be built in the near term. This will reduce the total reactor power by
about 25%, resulting in a reduction of $\sim\,$2.5 for $\Delta\chi^{2}$ with 6
years’ data.

  * •

The location of the JUNO experimental hall was shifted by about 60 m to adapt
to the underground geological conditions. The cosmic muon flux in the new
experimental hall increases by 30% due to a reduction of the vertical
overburden by 58 m. The changes on the baselines and the overburden have
negative but negligible impacts on the NMO sensitivity.

  * •

Backgrounds from the detector materials are under good control and are
improved with respect to the original expectation. Accidental backgrounds are
reduced to 0.7/day from 0.9/day.Detailss are shown in Section 3.11.

  * •

Recent efforts in optimization of the antineutrino selection show a
possibility to minimize the dead time after the muon veto. The live time
fraction increases from 83% to 93%, partially compensating for the statistics
loss due to less reactor power.

  * •

All 20-inch PMTs have been procured and tested. The average photon detection
efficiency for PMTs in the Central Detector is 29.1%, comparing to the
designed value 27% used in Ref. [9]. Meanwhile, a more realistic PMT and
liquid scintillator optical model has been developed with dedicated studies
using a Daya Bay detector [34], showing a possible photoelectron yield
increase and thus an improvement on the energy resolution.

  * •

Impacts of the energy non-linearity in the measured spectrum have been further
studied.

  * •

Combined analysis of the TAO and JUNO data shows that a moderate improvement
could be achieved compared to the simple assumption of 1% bin-to-bin
uncertainty, while the model dependence of the input spectrum [35, 36] can be
removed.

With all these factors taken into account, the NMO sensitivity is almost the
same as that in the Yellow Book.

In addition, as shown in Refs. [9, 37], electron antineutrino disappearance
and muon neutrino (antineutrino) appearance experiments will prefer a
different value of $\Delta m_{32}^{2}$ when the wrong NMO hypothesis is tested
in a common statistical analysis of the oscillation data. The corresponding
tension provides additional power to exclude the wrong NMO hypothesis and
greatly boosts the sensitivity of the NMO measurement. As a consequence, a
combined analysis between JUNO and an accelerator-based or atmospheric-based
NMO experiment will reach much better sensitivity than it may be expected from
a simple sum of $\Delta\chi^{2}$. The boosted $\Delta\chi^{2}$ due to the
combination effect of different types of experiments relies on the precisions
of $\Delta m_{31}^{2}$ (or $\Delta m_{32}^{2}$) from individual experiments.
The determination of $\Delta m_{32}^{2}$ with a high precision of 0.6% or
better (see Section 2.2) in JUNO experiment via reactor antineutrinos provides
unique information in the NMO combined analysis. Efforts towards the combined
analysis of JUNO+PINGU+ORCA [38] and JUNO+T2K+NO$\nu$A are also on-going.

###  2.2 Precision measurement of neutrino oscillation parameters

As illustrated in Fig. 1, JUNO will be the first experiment to observe the
effects of the so-called solar and atmospheric oscillations simultaneously.
The former, driven by $\Delta m^{2}_{21}$ and modulated by
$\sin^{2}2\theta_{12}$, cause the slow (low frequency) oscillation that is
responsible for the bulk of the reactor antineutrino disappearance. The
latter, driven by $\Delta m^{2}_{32}$ and modulated by $\sin^{2}2\theta_{13}$,
cause the higher frequency oscillations riding on top of the slower one.
Measuring the oscillated reactor antineutrino spectrum with a resolution of
$3\%$ at 1 MeV will enable determining these 4 parameters simultaneously.

An estimate of JUNO’s sensitivity to these 4 oscillation parameters was
previously reported in Ref. [9]. A new study is ongoing that incorporates
several important updates, namely:

  * •

Updated baselines and cosmogenic backgrounds that reflect the slight
adjustments made to the underground cavern’s position during construction.

  * •

An IBD cross-section that incorporates all radiative corrections and neutron
recoil effects.

  * •

A reactor antineutrino shape uncertainty consistent with the expectation from
the TAO experiment.

  * •

A data-driven scintillator non-linearity model inspired from the experience in
the Daya Bay experiment [39].

  * •

The updated energy resolution from Ref. [3].

  * •

The use of JUNO’s full Monte Carlo simulation to model detector response
effects such as non-uniformities and leakage.

  * •

The updated reactor configuration of Tab. 1.

The sensitivity to the oscillation parameters is being independently estimated
by several methods relying on different statistical approaches to treat the
uncertainties, such as pull parameters and covariance matrices, all of which
yield consistent results. The final values of the sensitivities are still
being finalized and will be the focus of a separate publication to be released
soon. While the precision on $\sin^{2}2\theta_{13}$ will not exceed that of
existing measurements [40], the $\sin^{2}2\theta_{12}$, $\Delta m^{2}_{21}$,
and $|\Delta m^{2}_{32}|$ parameters will be determined to $0.6\%$ or better
after 6 years of exposure, greatly improving over current estimates.

JUNO is currently the only experiment on the horizon that will determine some
of these parameters with such high precision. Moreover, its unique approach
using medium-baseline reactor antineutrino oscillations will feature very
different systematic uncertainties to those of other experiments, most notably
accelerator [15, 41, 42, 43] and solar [44, 45] experiments. As powerful
discriminators of neutrino masses and mixing models, constraints to other
experiments, and handles to probe the 3-neutrino flavor paradigm well beyond
current limits, these estimates will thus provide very valuable input to the
community for the foreseeable future.

###  2.3 Supernova neutrinos and trigger strategy

Neutrinos are crucial players during all stages of stellar collapse and
explosion. The detection of two dozen neutrino events from Supernova (SN)
1987A in Kamiokande-II [46], Irvine-Michigan-Brookhaven [47], and Baksan [48]
experiments, has essentially confirmed the scenario of the delayed neutrino-
driven explosion mechanism for core-collapse SNe [49, 50, 51, 52, 53].
However, with the poor statistics from SN 1987A, it is impossible to determine
supernova and neutrino parameters, although some hints can be obtained (see,
e.g., Ref. [54], for an overview on the statistical analysis of neutrino data
from SN 1987A). Future large water Cherenkov, liquid scintillator, and liquid
argon time projection chamber neutrino detectors (e.g., Hyper-Kamiokande,
JUNO, and DUNE) are all able to register large statistics of SN neutrinos and
have great potential to provide complete flavor information on SN neutrinos.
Meanwhile, numerical models have recently been advanced in better predictions
of the measurable SN neutrinos features, see recent reviews Refs. [55, 56].

Theoretical predictions of the SNe neutrino-emission depict three main phases,
namely the shock-breakout burst phase, post-bounce accretion phase and the
proto-neutron star cooling phase. It is highly desirable to obtain a clear
observation of these discriminative phases that correspond to the dynamical
evolution stages of stellar collapse and explosion. The total energy,
luminosity evolution, spectral distribution, and the mix of different flavors
of the SN neutrinos carry the information of the hydrodynamic conditions and
dynamical processes, the characteristics of the progenitor star and its
compact remnant. Moreover, before a massive star collapses and forms an SN,
i.e., during its silicon burning, a significant number of MeV-energy neutrinos
can be produced via thermal processes and nuclear weak interactions. These
pre-supernova neutrinos are also useful to investigate late-stage stellar
evolution [57, 58].

JUNO, with 20 kt LS, has excellent capability of detecting all flavors of the
$\mathcal{O}$(10 MeV) postshock neutrinos. The multichannel detection is
mainly via the inverse beta decay (IBD), $\overline{\nu}_{e}+p\to e^{+}+n$,
the elastic neutrino-electron scattering ($e$ES), $\nu+e^{-}\to\nu+e^{-}$, and
the elastic neutrino-proton scattering ($p$ES), $\nu+p\to\nu+p$. For a typical
galactic distance of 10 kpc and typical SN parameters, JUNO will register
$\sim$5000 IBD events, $\sim$300 $e$ES events and $\sim$2000 $p$ES events. The
charged-current (CC) and neutral-current (NC) interactions of neutrinos on 12C
nuclei are observable as well [59], and they will register $\sim$200 events
and $\sim$300 events, respectively. The 12B and 12N in the final states of CC
interactions on 12C are $\beta^{-}$-emitters with a 20.2 ms and 11 ms half-
life, respectively, leading to a prompt-delayed coincident signal. The visible
energy spectra caused by the six main reaction channels are shown in Fig. 4.
Note that the available numerical models in the community may predict quite
different signal statistics. The models from the Garching group [60] and Japan
group [61] predict $\mathcal{O}$(5k) IBD events at JUNO. The energy spectra of
$\overline{\nu}_{e}$, $\nu_{e}$, and $\nu_{x}$ (i.e., $\nu_{\mu}$,
$\nu_{\tau}$, and $\overline{\nu}_{\mu}$, $\overline{\nu}_{\tau}$) are of
crucial importance to understand the microscopic physics of SN explosions. A
unique strength of JUNO is the possibility to provide flux and spectral
information for the $\nu_{x}$ flavors based on the NC 12C and $p$ES channels,
respectively, as first pointed out in Ref. [62]. In fact, JUNO has the
capability to provide flux and spectral information for all flavors, as
demonstrated by a model-independent approach [63, 64].

Figure 4: The neutrino event spectra with respect to the visible energy
$E_{d}$ in the JUNO detector for a typical SN at 10 kpc. (Image remade from
Ref. [9])

The $\mathcal{O}$(1 MeV) pre-supernova neutrinos could provide a unique and
independent early warning for the optical observations of core-collapse SNe.
Attributing to the low detection threshold $\mathcal{O}$(100 keV) [65], pre-
supernova neutrinos are detectable via the IBD and $e$ES channels. However,
the $e$ES channel will suffer from the contaminations by natural radioactivity
and cosmogenic products, whereas the IBD channel is less affected. Therefore,
the pre-supernova neutrinos can be probed if the accumulated IBD rate
significantly exceeds the background level within a certain time window.
Limited pointing information becomes available once sufficient IBD events are
accumulated [66].

The JUNO front-end-electronics [67] will feature real-time waveform processing
by FPGAs (charge reconstruction and timestamp tagging) and a 2 GB DDR3 memory
shared by three PMTs. The processed signal will be sent to the data
acquisition (DAQ) in triggerless mode, while the raw waveforms will be sent to
DAQ once validated by the global trigger electronics. This configuration
limits the chance of data loss even in the face of the very high event rate
expected for a nearby supernova, because the supernova signals can be
temporarily stored in the 2 GB memory. It can completely handle the Supernova
explosion beyond 0.5 kpc (more than 2 M events). If a Supernova explosion is
even closer, the waveform processing by FPGA may start to have dead time and
the memory gets overflow. Furthermore, a dedicated multi-messenger trigger
system is under design to achieve an ultra-low detection threshold of
$\mathcal{O}$(10 keV). This system will enable JUNO to act as a powerful
transient machine for broad-band multi-messenger observation and connect to
the global network of multi-messenger observatories. JUNO can be expected to
become a major player in the next-generation Supernova Early Warning System
(SNEWS2.0) [68] for multi-messenger astronomy.

No SN neutrino burst has been observed since SN 1987A, and a recent analysis
that combines several independent studies yields an expected rate of $1.63\pm
0.46$ core-collapse SN per century [69]. The large neutrino detectors like
SuperK-Gd/JUNO/Hyper-K/DUNE will be online for several decades, so it seems
likely that we will obtain a high-statistics measurement of the neutrino
signal from at least one galactic core-collapse SN in the next few decades.
This will not only enable a deep understanding of the explosion mechanism, but
also probe the intrinsic properties of the neutrino themselves, e.g.,
constrain the absolute scale of neutrino masses [70]. Supernova neutrino
emission is predicted to be very variable. The detection of the integrated
signal from all past stellar core-collapse SNe, namely the diffuse SN neutrino
background, will improve understanding of the average SN neutrino signal and
the underlying cosmology, as discussed in the following section.

###  2.4 Diffuse supernova neutrino background

While core-collapse Supernovae (SNe) in our own galaxy are rare events, they
frequently occur throughout the visible Universe, sending bursts of neutrinos
in the direction of the Earth. They all contribute to a low background flux of
low-energy neutrinos, the so-called Diffuse Supernova Neutrino Background
(DSNB) on the level of $\sim 10\nu$ cm-2s-1 (e.g. [71]). Its exact flux and
spectrum bear information on the red-shift dependent supernova rate, average
SN neutrino energy spectrum and the fraction of black hole formation in core-
collapse SNe.

JUNO is in an excellent position to detect the $\bar{\nu}_{e}$ component of
the DSNB flux. Depending on the DSNB model, we expect about 2$-$4 IBD events
per year in the energy range above the reactor $\bar{\nu}_{e}$ signal. Given
the high light yield, the delayed signal from neutron capture on hydrogen in
the LS offers a very efficient tag for background reduction, while pulse-shape
discrimination helps to suppress the background from atmospheric-neutrino NC
interactions. This will provide JUNO with a DSNB sensitivity competitive to
the current Super-Kamiokande+Gadolinium phase.

####  2.4.1 DSNB signal

The energy spectrum of DSNB events in JUNO is given by

$\derivative{N_{\nu}}{E_{\nu}}=N_{p}\times\sigma_{\nu}(E_{\nu})\times
c\int_{0}^{\infty}\derivative{N(E^{\prime}_{\nu})}{E^{\prime}_{\nu}}\times\derivative{E^{\prime}_{\nu}}{E_{\nu}}\times
R_{\mathrm{SN}}(z)\times\left|\derivative{t}{z}\right|\differential z,$ (6)

where $N_{p}=7.16\times 10^{31}/(10\mbox{ }{\rm kton})$ is the number of
protons in the JUNO target, $\sigma_{\nu}(E_{\nu})$ is the energy-dependent
cross-section for the IBD reaction [72], and the last term represents the
differential DSNB flux. It is computed via a line-of-sight integral of the
average SN neutrino spectrum $\derivative{N}{E^{\prime}_{\nu}}$ (weighted by
an initial mass function), multiplied by the core-collapse SN rate
$R_{\mathrm{SN}}(z)$ that evolves over the cosmic history. To take into
account the effects of cosmic expansion, $E^{\prime}_{\nu}$ denotes the
neutrino energy at emission, while $E_{\nu}=E^{\prime}_{\nu}/(1+z)$ is the
red-shifted neutrino energy upon detection. The term $|\derivative{t}{z}|$
accounts for the expansion history of the Universe and relates $z$ to the
cosmic time $t$.

For the sake of simplicity, we assume the underlying SN neutrino spectra to
follow a Maxwell-Boltzmann distribution. The expected DSNB rate and spectra
largely depend on the expected mean energy $\langle E_{\nu}\rangle$. The left
panel of Fig. 5 displays an exemplary event spectrum for $\langle
E_{\nu}\rangle=15$ MeV. Event numbers are plotted as a function of the
detected prompt (i.e. positron) event energy and for 10 years of observation
in a 17-kton fiducial volume (see below). Tab. 4 displays the expected range
of the DSNB event rate depending on the average $\langle E_{\nu}\rangle$.

Figure 5: Prompt ($e^{+}$) energy spectra for the DSNB signal ($\langle
E_{\nu}\rangle=15$ MeV) and backgrounds as expected for JUNO. Left: After
basic selection cuts, the background from atmospheric-neutrino NC reactions
dominates over the whole range of the observation window from 10 to 30 MeV.
Right: When applying pulse shape discrimination, atmospheric neutrino NC and
fast neutron backgrounds are greatly reduced. The DSNB signal dominates in the
range from 11 to $\sim$22 MeV.

####  2.4.2 Background rejection

A variety of backgrounds besets the DSNB signal. The terrestrial flux of
$\bar{\nu}_{e}$’s from reactors and atmospheric neutrinos causes an
irreducible background and reduces the DSNB observation window to the range of
about 10 to 30 MeV.

Secondly, cosmic muons are penetrating the detector. In spallation processes,
these muons will generate $\beta/n$-emitting isotopes (mainly 9Li) in the
target material as well as fast neutrons when passing through the rock
surrounding the detector. Both can mimic the IBD signature but are effectively
reduced by a coincidence veto in the wake of a through-going muon and a
fiducial volume cut, respectively.

The most critical background for the DSNB search in LS detectors is created by
neutral-current (NC) interactions of atmospheric neutrinos with 12C [73, 74],
where the emission of one neutron together with a prompt energy deposit can
mimic the signature of IBD signals. To calculate the exclusive final-state
distribution of NC events, both the total cross-sections and residual nuclei’s
deexcitation processes have to be regarded. For this, the neutrino interaction
generator GENIE and the nuclear deexcitation tool TALYS have been used [75].
The corresponding event rates in the observation window are listed in the
first column of Tab. 4. The NC background rate surpasses the DSNB signal by
more than one order of magnitude. This is true for the whole spectral range
depicted in Fig. 5.

However, the intrinsic pulse-shape discrimination (PSD) capabilities of liquid
scintillators can be used to suppress the NC background to an acceptable
level. Here, we assume the PSD efficiencies as in the JUNO physics paper [76],
where 50% of the DSNB signal and 1.1% of the NC background will survive after
the PSD selection. The effect of the PSD on the signal and background rates
inside the observation window can be appreciated in the second column of Tab.
4. where a signal-to-background ratio $\gtrsim 1$ is achieved for all DSNB
models considered. The corresponding prompt energy spectra are shown in the
right panel of Fig. 5. The DSNB signal dominates in the range from 11 to
$\sim$22 MeV. Note that the residual atmospheric NC background rate may be
constrained in situ by measuring the radioactive decays of final-state nuclei,
bringing down the uncertainty on the background rate to a 10% level [77].

| Rate per 170 kton$\cdot$yrs
---|---
Source | w/o PSD | w/ PSD
DSNB signal |  $\langle E_{\bar{\nu}_{e}}\rangle$ = 12 MeV | 13 | 7
|  $\langle E_{\bar{\nu}_{e}}\rangle$ = 15 MeV | 23 | 12
|  $\langle E_{\bar{\nu}_{e}}\rangle$ = 18 MeV | 33 | 16
|  $\langle E_{\bar{\nu}_{e}}\rangle$ = 21 MeV | 39 | 19
Background | Reactor $\bar{\nu}_{e}$’s | 0.3 | 0.13
| Atmospheric CC | 1.3 | 0.7
| Atmospheric NC | 6$\times 10^{2}$ | 6.2
| Fast neutrons | 11 | 0.14
Table 4: Signal and background rates in the observation window (10$-$30 MeV),
assuming an exposure of 170 kton$\cdot$yrs. The first column shows raw event
rates, the second the residual rates after background rejection by pulse shape
discrimination (PSD).  Figure 6: Sensitivity to the DSNB signal vs. exposure,
depending on mean spectral energy and background uncertainty for which 20%
(10%) is assumed for solid (dashed) lines.

####  2.4.3 DSNB sensitivity

We determine the sensitivity of JUNO for a positive DSNB detection based on an
event counting analysis inside the observation window (i.e. neglecting
spectral information). The corresponding event numbers for 10 years of data
taking are summarized in the second column of Tab. 4. Fig. 6 illustrates the
sensitivity as a function of exposure. The sensitivity depends on the
uncertainty of the background rate assumed for the analysis. We expect a level
of 10% based on delayed tagging (see above) but show as well a more
conservative 20%. Sensitivity depends as well greatly on the mean spectral
energy $\langle E_{\bar{\nu}_{e}}\rangle$ and such event rate of the DSNB
signal. For $\langle E_{\bar{\nu}_{e}}\rangle\gtrsim 15$ MeV, JUNO can be
expected to provide 3$\sigma$ evidence of the DSNB signal after 10 years.

###  2.5 Solar neutrinos

Solar neutrinos are produced in thermonuclear fusion reactions in the solar
core and have played a significant role in the development of neutrino physics
and astrophysics. They represent a natural source of neutrinos with the energy
spectrum extending up to about 16 MeV. They can reveal direct information
about the solar core and can be exploited to study fundamental neutrino
properties and neutrino interactions with matter.

In the last decade, substantial experimental progresses have been made by
Borexino and Super-Kamiokande via high precision real-time spectroscopy of
solar neutrinos based on the elastic scattering (ES) off electrons. The energy
dependence of the electron neutrino survival probability $P_{ee}$ measured for
$pp$, 7Be, $pep$, and 8B neutrinos at Borexino [78] is compatible with the
transition from the so-called vacuum to the matter-dominated region predicted
by the MSW mechanism [79, 80]. Most recently, Borexino observed solar
neutrinos from the Carbon-Nitrogen-Oxygen (CNO) fusion cycle [81]. The world’s
largest sample of 8B solar neutrinos collected at Super-K made it possible to
study in detail the spectral distortion in the transition region of $P_{ee}$,
to constrain the neutrino oscillation parameters such as the mixing angle
$\theta_{12}$ and the mass-squared difference $\Delta m^{2}_{21}$, and to
measure the day-night asymmetry of the 8B neutrino interaction rate [45, 82].
A mild inconsistency is found between the value of $\Delta m^{2}_{21}$
reported by solar neutrino experiments and the KamLAND experiment.

There are still several open questions in solar neutrinos. Firstly, in
neutrino oscillation physics, the transition region of $P_{ee}$ is not fully
explored since it is difficult to reach an analysis threshold for ES electrons
to less than 3 (3.5) MeV in Borexino (Super-K). The exact shape of the
transition region is not only sensitive to $\Delta m^{2}_{21}$ value, but can
also be used to test new physics, such as the non-standard interactions, the
existence of sterile neutrinos, etc [83]. In addition, future high precision
measurements of $\sin^{2}\theta_{12}$ and $\Delta m^{2}_{21}$ based on solar
neutrinos can be used to test the consistency of the standard three neutrino
framework and to probe new physics beyond the Standard Model. On the other
hand, solar physics, and especially the question of solar metallicity, will
profit from more precise flux measurements of all neutrino components [9, 84].
However, any improvement of the flux measurement needs significant efforts in
the detector design, construction, and data analysis.

The JUNO experiment, using a Borexino-type liquid scintillator detector but
with a much larger mass comparable to the Super-K water Cherenkov detector,
has the potential to make significant contributions to the understanding of
solar neutrinos. Initial discussions on 7Be and 8B neutrinos were reported in
the Yellow Book [9]. The feasibility and physics potentials of detecting 8B
neutrinos have been further explored [85]. In this section, we will summarize
the recent 8B neutrino studies and discuss briefly the detection of 7Be and
$pp$ neutrinos. Sensitivity to the $pep$ and CNO solar neutrinos is hindered
by the production of 11C cosmogenic background, since the JUNO overburden is
relatively shallow, for example, comparing to the Gran Sasso underground
laboratory where Borexino is located.

####  2.5.1 Measurement of 8B solar neutrinos

For JUNO as an LS detector, the primary detection channel for solar neutrinos
is the elastic scattering off electrons. The background consists of the
intrinsic natural radioactivity in LS, gamma rays from external detector
materials, and unstable isotopes produced by cosmic ray muons passing through
the detector. With sufficient shielding, the external background with energies
larger than 2 MeV is suppressed to a negligible level. This permits the
reduction of the analysis threshold to 2 MeV in a spherical fiducial volume
with a radius of 13 m. However, the threshold can not be reduced further due
to numerous cosmogenic 11C decays, more than 10,000 per day in the LS. Above
the 208Tl decay spectrum, the fiducial volume is further enlarged. Muon-
induced backgrounds are well suppressed by cylindrical volume veto cuts along
all muon tracks, and spherical volume veto cuts around spallation neutron
candidates. Assuming a 10-17 g/g level for the intrinsic 238U and 232Th
contamination, the signal and background spectra are shown in Fig. 7. In ten
years of data taking, after all cuts are applied, about 60,000 signal events
and 30,000 background events are expected in the energy range above 2 MeV.
Details are reported in Ref. [85].

Figure 7:  Left: signal and background spectra above 2 MeV expected in JUNO in
10 years after the selection cuts. The energy-dependent fiducial volumes
account for the discontinuities at 3 MeV and 5 MeV. Right: the expected
precision of $\sin^{2}\theta_{12}$ and $\Delta m^{2}_{21}$ using 8B solar
neutrinos and reactor antineutrinos at JUNO. Both plots are taken from Ref.
[85]

Neutrino oscillation physics would benefit from a large low-threshold ES
sample of 8B neutrinos. For example, JUNO can set a statistical limit for
rejecting the absence of an upturn in the $P_{ee}$ transition region of
5$\sigma$ if $\Delta m^{2}_{21}=7.5\times 10^{-5}$ eV2. Taking systematic
uncertainties into consideration, a $\sim$3$\sigma$ sensitivity is achievable.
Moreover, the expected precision of the day-night asymmetry measurement is
0.9%, slightly better than the most recent result (1.1%) of Super-K [82]. In
the standard neutrino oscillation framework, a joint fit of the ES reaction
rate, the spectral distortion, the day-night asymmetry enables a simultaneous
determination of $\sin^{2}\theta_{12}$ and $\Delta m^{2}_{21}$, as shown in
Fig. 7. Here, the arrival flux of 8B neutrinos is constrained to
(5.25$\pm$0.20)$\times 10^{6}$ cm-2s-1 according to the NC measurement of SNO
[44], and the flux of $hep$ neutrinos is fixed to 8.25$\times 10^{3}$ cm-2s-1
[86]. The 8B neutrino spectrum and its uncertainty are taken from Ref. [87].
In the fitting results, the precision of $\sin^{2}\theta_{12}$ is limited by
the neutrino flux uncertainty in the SNO measurement. Although limited by the
statistical uncertainty of the day-night asymmetry measurement, the $\Delta
m^{2}_{21}$ precision is similar to the solar global fitting results in Ref.
[88].

In addition to the ES channel, the possible CC and NC channels between
neutrinos and carbon nuclei have been explored in Refs. [89, 90, 91]. The
primary channels and the expected event numbers for a 200 kt$\cdot$year
exposure are listed in Tab. 5. The reactions between neutrinos and 12C nuclei
can not happen due to high energy thresholds. The 1.1% natural abundance of
13C makes it possible to measure the 8B neutrino flux via both the CC and NC
channels. There are 9$\times 10^{30}$ 13C nuclei in a 20 kt LS. Using the
cross-sections in Ref. [91], about 6,000 CC signals and 3,000 NC signals are
expected in 10 years of data-taking.

In the CC reaction, the electron carries most of the neutrino energy, but it
should be separated efficiently from the ES electrons. For reaction 3 in Tab.
5, the CC electrons could be distinguished by tagging the subsequent 13N
decays. In the NC reaction, all neutrino flavors have the same cross section.
For reaction 5, the 3.685 MeV $\gamma$’s from the de-excitation of
${}^{13}{\rm C}^{*}$ could form a small peak on the ES electron spectrum.
Benefiting from the excellent energy resolution, the area of the $\gamma$ peak
can be obtained from a joint fitting with the continuous spectrum of ES
electrons. The sensitivity would further profit from the efforts of
discriminating $\gamma$ and $e^{-}$ in Ref. [92]. Detailed studies will be
reported later.

| Type | Reaction | Threshold | Final products |  8B | $hep$
---|---|---|---|---|---|---
1 | ES | $\nu+e^{-}\rightarrow\nu+e^{-}$ | 0 | $e^{-}$ | 3$\times 10^{5}$ | 640
2 | CC | $\nu_{e}+^{12}{\rm C}\rightarrow e^{-}+^{12}{\rm N}$ | 16.8 MeV | $e^{-}$ | 0 | 0.41
3 | CC | $\nu_{e}+^{13}{\rm C}\rightarrow e^{-}+^{13}{\rm N}$ | 2.2 MeV |  $e^{-}$ and 13N decay | 3768 | 14
4 | NC | $\nu+^{12}{\rm C}\rightarrow\nu+^{12}{\rm C}^{*}~{}(1^{+})$ | 15.1 MeV | $\gamma$ | 0.2 | 5
5 | NC | $\nu+^{13}{\rm C}\rightarrow\nu+^{13}{\rm C}^{*}~{}(\frac{3}{2}^{-})$ | 3.685 MeV | $\gamma$ | 3165 | 13.5
Table 5:  Reactions via ES, CC, and NC channels for 8B and $hep$ solar
neutrinos in JUNO. The statistics correspond to a 200 kt$\cdot$year exposure
and a 100% detection efficiency.

####  2.5.2 Potential of measuring $pp$ and 7Be neutrinos

The Compton-like edge (665 keV) of the continuous spectrum of scattered
electrons due to 7Be solar neutrinos is a very distinct feature of the energy
spectrum. Combined with the unprecedented energy resolution of JUNO, it should
be possible to detect 7Be solar neutrinos considering a wide range of
background levels. With the optimal assumption of 238U and 232Th
concentrations in Ref. [9], i.e., 10-17 g/g in secular equilibrium, these
contaminations can be determined via easily detectable fast
214(212)Bi/214(212)Po coincidences, respectively. From the experience of
Borexino [81], 210Po and 210Pb/210Bi isotopes that are part of the 238U chain
could have additional sources and break the secular equilibrium. For 210Po, it
is possible to identify such $\alpha$’s on an event-by-event basis with high
efficiency and purity [92]. In addition, 210Po has a lifetime of 200 days and
thus its contamination can be significantly reduced over the whole JUNO
lifetime. However, the contamination of 210Bi (Q = 1160 MeV, $\tau$ = 7.23
days), supported by the long-lived 210Pb (Q = 63 keV, $\tau$ = 32.2 years) can
be disentangled only via a spectral fit of the data itself. Similarly
dangerous background is 85Kr (Q = 687 keV, $\tau$ = 15.4 years) that is
present in the air. Even though its electron spectrum is correlated with 7Be
neutrino spectrum, 85Kr contamination can be constrained via its rare decay
branch (0.43%) providing fast coincidences [93]. Due to these possible
backgrounds, exceeding the 2.7% precision of the flux measurement of 7Be
neutrinos achieved in Borexino Phase-II [78] will be challenging and needs
further studies. The measurement of 7Be neutrino flux can be useful for
solving the “metallicity problem” in solar physics, searching for neutrino
magnetic moment [94] and flavor-diagonal non-standard neutrino interactions
[95]. Ideas on searching for the 7Be neutrino flux time variations at
different frequencies have also been presented [96].

Measurement of the $pp$ neutrinos with the highest flux and lowest energy ($<$
420 keV) is challenging due to the irreducible background from 14C (Q = 156
keV, $\tau$ = 8270 years). Even in the ideal case of JUNO achieving the radio-
purity of Borexino Phase-I, the $pp$ neutrino signal would be the dominating
part of the spectrum only in a narrow interval between 160 to 230 keV.
Additional complications arise from the random pile-up of 14C with itself or
with other backgrounds [97]. The energy resolution at low energies would also
be affected by the relatively high dark noise of the 20-inch PMTs. All these
effects will need careful studies. Borexino has measured the $pp$ neutrino
flux with about 10% precision [78]. A more precise $pp$ measurement would
improve the comparison between the solar photon and neutrino luminosities and
would help to examine the energy dependence of neutrino oscillation in the
vacuum-dominated region.

###  2.6 Atmospheric neutrinos

Atmospheric neutrinos are a very important source for studying neutrino
oscillation physics. In 1998, the Super-Kamiokande experiment reported the
first evidence of neutrino oscillation based on a zenith-angle dependent
deficit of atmospheric muon neutrinos [98]. Atmospheric neutrinos originate
from the decays of $\pi$ and $K$ produced in extensive air showers initiated
by the interaction of cosmic rays with the Earth’s atmosphere [99, 100]. Since
the Earth is mostly transparent to neutrinos below the PeV energy scale, an
atmospheric neutrino detector is able to see neutrinos coming from all
directions. They have a broad range in baseline (15 km $\sim$ 13000 km) and
energy (0.1 GeV $\sim$ 10 TeV) and contain neutrinos and antineutrinos of all
flavors. The Mikheyev-Smirnov-Wolfenstein (MSW) matter effect [79, 80] acting
on neutrinos passing through the Earth will play a key role in determining the
Neutrino Mass Ordering (NMO). It is worthwhile to stress that the JUNO NMO
sensitivity from atmospheric neutrinos is complementary to that from the JUNO
reactor neutrino results. Since the best fit occurs at different values of
$|\Delta m_{31}^{2}|$ for the wrong-ordering hypothesis, the combined
sensitivity from atmospheric and reactor neutrinos will exceed the purely
statistical combination of their stand-alone sensitivities [37, 38].

In Ref. [9], we have investigated atmospheric neutrinos in JUNO and discussed
their contributions to the NMO. In terms of the reconstruction potential of
the JUNO detector, we conservatively use the atmospheric $\nu_{\mu}$ and
$\bar{\nu}_{\mu}$ events with the track length $L_{\mu}$ larger than 5 m and
$1^{\circ}$ angular resolution. These events have been classified into the
fully contained (FC) $\nu_{\mu}$-like, FC $\bar{\nu}_{\mu}$-like, partially
contained (PC) $\nu_{\mu}$-like and PC $\bar{\nu}_{\mu}$-like samples in terms
of the $\mu^{\pm}$ track and the statistical charge separation. Our numerical
results have shown that the JUNO’s NMO sensitivity can reach 0.9$\sigma$ for a
200 kton-years exposure and $\sin^{2}\theta_{23}=0.5$. In addition, an
optimistic sensitivity may reach 1.8$\sigma$ for 10-year data based on the
following reasonable assumptions. Firstly, the $\nu_{e}/\bar{\nu}_{e}$ CC
events can be identified and reconstructed very well if the $e^{\pm}$ visible
energy $E^{e}_{\rm vis}>1$ GeV and the visible energy of accompanying hadrons
is smaller than $E^{e}_{\rm vis}$. Secondly, we extend the selection condition
$L_{\mu}>5$m to $L_{\mu}>3$m for the $\nu_{\mu}/\bar{\nu}_{\mu}$ CC events.
Finally, the charged lepton direction will be replaced by the neutrino
direction with a $10^{\circ}$ angular resolution. A combined NMO sensitivity
with both atmospheric and reactor neutrinos in JUNO will be evaluated.

In Ref. [101], JUNO’s performance in reconstructing the atmospheric neutrino
spectrum has been evaluated. A Monte Carlo sample of the atmospheric neutrinos
has been generated, the simulated spectrum has been reconstructed between 100
MeV and 10 GeV, showing a great potential of the detector in the atmospheric
low energy region. The different hit time patterns caused by final-state
electrons and muons allow discriminating the flavor of the primary $\nu_{e}$
and $\nu_{\mu}$ neutrinos. To reconstruct the time pattern of events, the
signals of 3-inch PMTs have been used, which will be operated in digital mode
due to their small area. Furthermore, the Transit Time Spread (TTS) of 3-inch
PMTs is of the order of the nanosecond, while the 20-inch PMTs one is larger,
for most of them. By use of a probabilistic unfolding method, we have
reconstructed separately the primary $\nu_{e}$ and $\nu_{\mu}$ spectra as
shown in Fig. 8. To remove low-quality events, preliminary cuts have been
applied to the neutrino sample. A cut on the interaction vertex position is
applied, to remove events that release their energy near the edge of the
acrylic vessel. Furthermore, an additional cut on the total number of hits
seen by the water pool veto PMTs has been used in the analysis to discard PC
events and suppress the atmospheric muon background. The resulting $\nu_{e}$
and $\nu_{\mu}$ populations have been passed through the analysis procedure.
The total uncertainty on the atmospheric neutrino spectrum reconstruction is
evaluated in each energy bin, including both contributions from statistics and
systematic effects, and it is reported in Fig. 8. The effects from oscillation
parameters and cross-section uncertainties, sample selection and unfolding
procedure have been included in estimating the systematics. The total
uncertainty ranges between $10\%$ and $25\%$, with the best performance
obtained at around 1 GeV.

Figure 8: Atmospheric neutrino energy spectra reconstructed by the JUNO
detector for $\nu_{\mu}$ (blue color) and $\nu_{e}$ (red color), compared with
present Super-Kamiokande [102] and ${\rm Fr\acute{e}jus}$ [103] measurements
in the same energy region. The HKKM14 [104] model predictions are also
reported, both at the source and including oscillation effects. The fluxes are
multiplied by $E^{2}$ to give a better picture.

###  2.7 Geoneutrinos

Geoneutrinos, antineutrinos from the decays of long-lived radioactive elements
inside the Earth, are a unique tool to study our planet, particularly its
radiogenic power, and bring insights into its formation and chemical
composition. The inverse beta decay on protons with 1.8 MeV threshold makes it
possible to measure geoneutrinos from the 238U and 232Th decay chains. Only
two experiments have measured geoneutrinos so far: KamLAND [105, 106, 107,
108, 109] and Borexino [110, 111, 112, 113]. Both experiments reached a
precision of about 16-18%, detecting in total $168.8^{+26.3}_{-26.5}$ events
in KamLAND [109] and $52.6^{+9.4}_{-8.6}\,({\rm stat})^{+2.7}_{-2.1}\,({\rm
sys})$ events in Borexino [113]. Both experiments do not have sufficient
sensitivity to determine the Th/U ratio, a parameter important for
understanding the Earth’s formation, with good accuracy.

To facilitate the comparison among experiments, the normalized $\bar{\nu_{e}}$
geoneutrino signal rate is expressed in Terrestrial Neutrino Units (TNU)111One
TNU corresponds to 1 event detected with a detector of 100% detection
efficiency, containing 1032 target protons (roughly 1 kton of liquid
scintillator) in 1 year.. A total signal of $47.0^{+8.4}_{-7.7}\,({\rm
stat)}^{+2.4}_{-1.9}\,({\rm sys})$ TNU is measured by Borexino [113]. The
null-hypothesis of a mantle signal is excluded at 99% C.L. The estimated Earth
radiogenic power from U and Th is $31.7^{+14.4}_{-9.2}$ TW. Borexino sets
tight upper limits on the power of a georeactor hypothesised [114, 115, 116,
117] to be present inside the Earth. KamLAND measured [109] geoneutrino signal
of $32.1^{+5.0}_{-5.0}$ TNU and estimated the Earth radiogenic power from U
and Th as $12.4^{+4.9}_{-4.9}$ TW. Contrary to Borexino, KamLAND prefers the
geological models with a lower concentration of heat-producing elements in the
Earth, which brings a weak discrepancy between the two results.

A next-generation experiment like JUNO is needed to provide more precise
results concerning the Earth’s radiogenic power and to fully exploit
geoneutrinos in order to understand the natural radioactivity of our planet.
With its detector being at least 20 times larger than the existing detectors,
JUNO will join the family of geoneutrino experiments and represent a fantastic
opportunity to measure geoneutrinos. Within the first year of running, JUNO
will record more geoneutrino events than all other detectors will have
accumulated to that time. In the following, we predict the signal and the
precision level of JUNO and introduce a local refined geological model. These
models provide a possibility of extracting the mantle component.

####  2.7.1 Potential of geoneutrino measurement in JUNO

The JUNO detector will provide an exciting opportunity to obtain a high
statistics measurement of geoneutrinos. The expected signal and background
rates, corresponding systematic uncertainties, and the JUNO sensitivity to
geoneutrinos are published in [118, 9].

The expected geoneutrino signal depends on U and Th abundances and
distribution in the Earth. The published JUNO sensitivity studies [118, 9]
consider a global composition of the Earth as described in Ref. [119], leading
to an expected signal in JUNO of $39.7^{+6.5}_{-5.2}$ TNU ($\sim$400
geoneutrinos per year). The 500 km range of the crust around JUNO contributes
more than 50% to the total signal [119]. Thus, local refined geological models
are needed for a precise estimation of the crustal signal and, consequently,
for disentanglement of the mantle signal. Such an effort is also a good
opportunity for geologists and particle physicists to work together and test
our predictions of the planet’s interior.

The main challenge for the JUNO geoneutrino measurement is the large reactor
antineutrino background. In addition, there are other non-antineutrino
backgrounds relevant for geoneutrino detection, like cosmic muons, cosmogenic
9Li-8He, fast neutrons,13C$(\alpha,n)^{16}$O reactions, and accidental
coincidences. The expected energy spectrum is shown in Fig. 9.

Figure 9: (a) The energy spectra of prompt IBD candidates from geoneutrinos
(red), reactor antineutrinos (orange), and other non-antineutrino backgrounds
(accidental in blue, 9Li-8He in green,13C$(\alpha,n)^{16}$O in small magenta)
for one year of data-taking after the selection cuts [9]. (b) The expected
1$\sigma$ uncertainty for geoneutrino measurement in JUNO with a fixed
chondritic Th/U ratio as a function of live time [118].

The potential of the JUNO geoneutrino measurement was estimated by two methods
considering the same muon veto (17% exposure loss), IBD selection (80%
efficiency), and detector response (3% at 1 MeV energy resolution). One is
based on the generation of thousands of pseudo-experiments, in which the rate
of each spectral component is randomly extracted from the Gaussian
distributions reflecting the uncertainty of rate predictions [9]; each pseudo-
experiment is then fit using a log-likelihood method and constraining the non-
antineutrino backgrounds. The other method is using the Asimov data set, which
employs the $\chi^{2}$ fit method to quantitatively assess the potential of
geoneutrino measurements in JUNO [118]. The theoretical spectral shapes are
used for geoneutrinos [120]. The first method did not consider the shape
uncertainty of spectral components, while the second one did. The two methods
gave compatible results. After 1, 3, 5, and 10 years, the first (second)
method estimated the precision of geoneutrino measurement with a fixed
chondritic mass Th/U = 3.9 in the fit as 17(13)%, 10(8)%, 8(6)%, and 6(5)%.
The results of the second method are shown in Fig. 9. After several years of
measurements, JUNO also has the potential to constrain the Th/U ratio in the
observed geoneutrino signal [118, 9].

####  2.7.2 Refined crustal models and expected signal for JUNO

The fact that geoneutrinos signal produced by U and Th in the continental
crust within $\sim$100 km of the detector equals the whole mantle signal [119]
illustrates the importance of refined geological models around JUNO. Involving
the advanced know-how of the geophysical and geochemical communities, the
South China Block surrounding JUNO can be characterized in terms of density,
crustal layers thickness, and chemical composition.

The JULOC (JUNO Local Crust) [121] and GIGJ (GOCE Inversion for Geoneutrinos
at JUNO) [122] models provide information about the thickness and density of
the upper, middle, and lower crust around JUNO. JULOC, a 3-D comprehensive
high-resolution crustal model around JUNO, uses seismic ambient noise
tomography [123] in the input and covers an area of $10^{\circ}\times
10^{\circ}$ around the detector. For the South China block, the crustal
structure is pretty uniform. No significant large-scale velocity and density
anomalies are found in the JULOC model. The density uncertainty is less than
that of the global crustal model. According to GIGJ, a $6^{\circ}\times
4^{\circ}$ refined geophysical model obtained by a Bayesian inversion of
gravimetric data of GOCE satellite (Gravity field and steady‐state Ocean
Circulation Explorer), the overall thickness uncertainties of upper, middle,
and lower crustal layers are 2%, 3%, and 1%, respectively [122].

On the basis of compiled U and Th abundances data from over 3000 rock samples,
the JULOC model suggests that the local upper (middle and lower) crust has
higher (lower) average U and Th abundances than the global average. The
estimated crustal geoneutrino signal by JULOC is ($38.3\pm 4.8$) TNU, to be
compared with ($28.2^{+5.2}_{-4.5}$) TNU [119] based on a global crustal model
[124]. The significant increase of the signal is mainly the consequence of the
upper crust of South China Block that is richer in U and Th than the global
average.

###  2.8 Nucleon decays

The baryon number $B$ violation is one of three basic ingredients to explain
the observed cosmological matter-antimatter asymmetry [125]. The baryon number
is necessarily violated in the Grand Unified Theories (GUTs) [126, 127], which
can unify the strong, weak, and electromagnetic interactions into a single
underlying force at a scale of $M_{GUT}\simeq 2\times 10^{16}$ GeV. A general
prediction of the GUTs is nucleon decay. However, no experimental evidence to
date for the proton decay or $B$-violating neutron decay has been found. With
a 20 kton LS detector, JUNO is expected to have a good performance on the
nucleon decay search, especially on the channel of $p\to\bar{\nu}K^{+}$ [128,
9]. This decay mode displays a clear three-fold coincidence feature in time, a
prompt signal from the $K^{+}$ kinetic energy, a short delayed signal
($\tau=12.38$ ns) from its decay daughters, and a long-delayed signal
($\tau=2.2$ $\mu$s) from the final Michel electron.

The JUNO potential on $p\to\bar{\nu}K^{+}$ using the JUNO Monte Carlo (MC)
simulation framework [129] and considering the detector performances in
detail. Generations of $p\to\bar{\nu}K^{+}$ and atmospheric neutrino
backgrounds rely on GENIE 3.0 [130]. We modified the GENIE code to take into
account the final state interaction and the residual nucleus de-excitations.
The time-correlated triple coincidence permits the JUNO detector to
effectively identify the $p\rightarrow\bar{\nu}K^{+}$ signals and to reject
atmospheric neutrino backgrounds. The primary production $K^{+}$ and its decay
daughters will leave two deposition signals in the hit-time spectrum after TOF
correction. There will be a pile-up of signals due to the short lifetime of
$K^{+}$. With the help of the 3-inch PMT system (TTS 1.5 ns) of JUNO, we can
observe this pile-up shape in the hit time spectrum, as shown in Fig. 10. Due
to this feature, the uprising time of $p\to\bar{\nu}K^{+}$ should be larger
than that of atmospheric neutrinos [131]. Furthermore, the selection of
$p\to\bar{\nu}K^{+}$ can be improved with the help of multi-pulse fitting
[132], in which we can reconstruct both signals, and get the correlated time
difference and the energy deposition of each signal, as shown in the right
panel of Fig. 10.

Figure 10: Left: A sample of hit time distribution after TOF correction. A
double signal pile-up shape should be observed. The third signal in around 500
ns comes from the Michel election. Right: A sample of Multi-pulse fitting. The
time difference of both signal is 11 ns.

Beyond the timing analysis, additional selection criteria have been considered
using visible energy, fiducial volume, muon veto, the Michel electron, and
neutron capture. Using all these criteria, 31% $p\to\bar{\nu}K^{+}$ remained
while only 0.3 backgrounds survived in ten years. According to the Feldman-
Cousins approach [133], the preliminary sensitivity of $p\to\bar{\nu}K^{+}$ of
JUNO is estimated as $8.34\times 10^{33}$ years (90% C.L.) for JUNO 10 years
running. In addition to $p\to\bar{\nu}K^{+}$, other nucleon decay modes, such
as $n\rightarrow 3\nu$ and $p\rightarrow\mu^{+}\mu^{+}\mu^{-}$ are being
investigated; it is expected that JUNO will give more stringent limits on
these decay channels in the future.

###  2.9 Other physics

JUNO has the potential to address a wide range of open questions in the fields
of particle physics, nuclear physics, astrophysics and cosmology besides the
topics covered in the previous parts. In this subsection, we shall explore
several new directions, which are of fundamental importance in the field of
particle physics and cosmology.

  * •

Testing the Majorana nature of the neutrino is one of the most important tasks
in the field of particle physics and nuclear physics. The only realistic way
to probe this is the search for the neutrinoless double beta
($0\nu\beta\beta$) decay ${{}^{A}_{Z}}N\to{{}^{A}_{Z+2}}N+2e^{-}$ [134], in
which the half-life is possibly reachable based on the effective neutrino mass
$m_{\beta\beta}$ constrained by the mass splitting measurements in neutrino
oscillation, if it is mediated by light active Majorana neutrinos.
Experimental studies of the $0\nu\beta\beta$ decays using ${}^{76}{\rm Ge}$,
${}^{136}{\rm Xe}$ or ${}^{130}{\rm Te}$ have been carried out in the past
years, and the lower limits on the $0\nu\beta\beta$ half-life have reached the
levels of $10^{25}~{}{\rm}$ to $10^{26}~{}{\rm yr}$ [134], corresponding to an
effective neutrino mass $m_{\beta\beta}$ of $(60\cdots 300)~{}{\rm meV}$
depending on the values of nuclear matrix elements. Next-generation ton-scale
$0\nu\beta\beta$ decay experiments will improve the half-life sensitivity to
the levels of $10^{27}~{}{\rm}$ to $10^{28}~{}{\rm yr}$ [135, 136], covering
the $m_{\beta\beta}$ range of $(10\cdots 50)~{}{\rm meV}$ for the inverted
mass ordering.

However, the parameter space for the normal mass ordering is beyond the
sensitivity of next-generation experiments. Suitable experimental strategies
are still under development. The JUNO experiment has great potential to be
upgraded to a $0\nu\beta\beta$ phase [137] after the primary goal of the
neutrino mass ordering is accomplished. JUNO’s sensitivity reaches the meV
scale. Preliminary studies have shown that, within 5 years the
$m_{\beta\beta}$ sensitivity could reach $(5\cdots 12)~{}{\rm meV}$ with 50
tons of fiducial ${}^{136}{\rm Xe}$ [138], and $(2.3\cdots 6.0)~{}{\rm meV}$
with 400 tons of fiducial ${}^{130}{\rm Te}$ [139], respectively. The
effective energy resolution near the $Q$ values is at the level of 2%. The
significant backgrounds arise from ${}^{8}{\rm B}$ solar neutrinos, two-
neutrino double beta decays, natural radioactivity, and cosmogenic
radionuclides. It is shown in Ref. [138] that the background index could be
comparable to the next generation liquid Xeon experiment. A more careful
evaluation of the expected background levels is required and the investigation
of isotope doping and background reduction is still on-going.

Finally, one should emphasize the importance for future $0\nu\beta\beta$ decay
experiments to reach the sensitivity of $m_{\beta\beta}$ around $1~{}{\rm
meV}$. Such a measurement can not only probe the Majorana nature of neutrinos
and lepton number violation, but also determine the lightest neutrino mass,
fix the neutrino mass spectrum and place restrictive constraints on one of two
Majorana CP phases. All of these go beyond the expected sensitivities of
future $\beta$ decay experiments [140, 141] and cosmological observations
[142].

  * •

The existence of Dark Matter (DM) has been well confirmed in the scales of the
galaxies, galaxy clusters and cosmological distance. However, the particle
nature of DM is still an open problem in the field of particle physics and
cosmology. Neutrinos produced from the annihilation or decays of DM inside the
Sun or Galactic Centre are a promising indirect signature of DM, and have been
searched for in large neutrino detectors, such as Super-Kamiokande [143] and
IceCube [144]. Sensitivity studies on the neutrino signals from the DM
annihilation inside the Sun have been obtained for JUNO using electron
neutrinos [145] and muon neutrinos [9]. Within ten years of data taking,
sensitivities for the spin-independent cross-section can reach the level of
$(10^{-41}\cdots 10^{-42})~{}{\rm cm}^{2}$ for both electron and muon neutrino
channels. These sensitivity levels will be relevant for the DM mass smaller
than 10 GeV. On the other hand, JUNO can provide a sensitivity at the level of
$(10^{-39}\cdots 10^{-40})~{}{\rm cm}^{2}$ for DM-nucleon spin-dependent cross
section [9, 145], which will surpass the existing limits for light DM
particles.

Putative primordial black holes (PBH) are a viable candidate for the cold DM
[146], and detectable in the mass range of $(10^{15}\cdots 10^{16})~{}{\rm g}$
via neutrinos and antineutrinos that are emitted as part of the Hawking
radiation [147]. A search for PBH via low energy antineutrinos [148] has been
carried out using Super-Kamiokande data [149]. JUNO can improve the current
limit on PBH abundance by one order of magnitude [150] because of its
excellent neutron tagging and background reduction techniques.

  * •

Light sterile neutrinos [151], non-unitarity of the neutrino mixing matrix
[152] and non-standard neutrino interactions [153] are representative new-
physics scenarios beyond the standard three neutrino oscillation framework.
JUNO can search for light sterile neutrinos in several sensitive oscillation
channels. Precise reactor antineutrino measurements of the JUNO detector can
set severe limits on light sterile neutrinos for a $\Delta m^{2}$ range of
$(10^{-5}\cdots 10^{-3})~{}{\rm eV}^{2}$ [9]. Moreover, the JUNO-TAO detector
will be able to test the reactor antineutrino anomaly and search for light
sterile neutrinos at the eV scale [20]. Precision measurement of oscillation
parameters at JUNO, in particular for the mixing angle $\theta_{12}$, is
crucial in the global effort of leptonic unitarity tests [9, 154]. Finally
different completive limits for new physics beyond three-flavor oscillations
can be obtained with the high-precision oscillation measurements of reactor
antineutrinos [155, 156], solar neutrinos [157], atmospheric neutrinos [158,
159] and supernova neutrinos [160].

  * •

Large neutrino detectors also are excellent tools to observe exotic particles
produced in cosmic-ray interactions or the early Universe. The magnetic
monopole was first predicted by Dirac in 1931, and is a general prediction of
the Grand Unified Theories [161]. Searches for signals of magnetic monopoles
have been performed by many experiments, such as MACRO [162], Soudan-2 [163]
and NO$\nu$A [164]. Q-balls [165] are predicted as non-topological solitons in
theories with a global $U(1)$ symmetry and a complex scalar field, which can
act as a viable candidate of DM [166]. They have been searched for by several
experiments, including Super-Kamiokande [167], Baksan [168] and IceCube [169].
Strange Quark Matter (SQM) [170] with approximately equal numbers of up, down
and strange quarks is predicted as the ground state of quantum chromodynamics.
Stable SQM objects with masses larger than $10^{10}$ GeV are known as
nuclearites [171]. Searches for nuclearites have been carried out in MACRO
[172], SLIM [173], and ANTARES [174], with no experimental evidence. All these
exotic particles can be searched for using the JUNO detector with rather
similar detection strategies. Taking the nuclearites for instance, it has been
shown that the sensitivity can be improved by one order of magnitude for the
masses of nuclearites between $10^{13}$ GeV and $10^{15}$ GeV [175].

Many more exciting topics in JUNO with leading sensitivities or unique
features might be added to the list, given the large target mass, low
background, low energy threshold and excellent energy resolution of the
detector.

##  3 Detector design and R&D

###  3.1 Detector design overview

The JUNO detector system includes a 20 kton liquid scintillator (LS) detector,
a water Cherenkov detector in which the liquid scintillator detector is
submerged, and a plastic scintillator array on top of them. A schematic view
of the detector is shown in Fig. 3.

The 20 kton LS is contained in a spherical acrylic vessel with an inner
diameter of 35.4 m. The Main Structure, including a spherical stainless steel
structure and its bearing, supports the acrylic vessel via 590 connecting
bars. The light emitted by the LS is watched by 17,612 20-inch PMTs (referred
to as “large PMT”) and 25,600 3-inch PMTs (referred to as “small PMT”), which
are installed on the inner surface of the Main Structure. The photocathode
coverage is 75.2% for the 20-inch PMTs and 2.7% for the 3-inch PMTs. All PMTs
have been tested. The average photon detection efficiency is 29.1% for large
PMTs in the Central Detector and $>24\%$ for small PMTs. The large PMTs
feature special protection in case of implosion. The entire LS detector is
submerged in a cylindrical water pool. A water buffer of 1.42 m thickness
between the acrylic vessel and the PMT surface protects the LS from the PMT
glass’s radioactivity. The water buffer is connected with the outer water
Cherenkov detector but is optically separated. A chimney for calibration
operations is connected to the top of the acrylic vessel. Special
radioactivity shielding and a muon detector will cover the chimney and the
calibration system on the top.

The LS has a similar recipe as the Daya Bay LS but without gadolinium loading.
Linear alkylbenzene (LAB), a straight alkyl chain of 10–13 carbons attached to
a benzene ring, is used as the detection medium due to its excellent
transparency, high flash point, low chemical reactivity, and good light yield.
The JUNO LS will contain 2.5 g/L 2,5-diphenyloxazole (PPO) as the fluor and 3
mg/L p-bis-(o-methylstyryl)-benzene (bis-MSB) as the wavelength shifter. The
composition has been optimized in dedicated studies with a Daya Bay detector
[34, 176]. The LS will be purified before filling to improve its radiopurity
and transparency, which is crucial for such a gigantic detector. A pilot plant
test shows that the LS attenuation length can reach $>20$ m. Combined with the
PMT detection efficiency and other detector parameters, a yield of 1345
photoelectrons per MeV is obtained in simulations. The targeted LS radiopurity
of JUNO is $10^{-17}$ g/g for U/Th/K.

The water pool has a diameter of 43.5 m and a height of 44 m, providing
sufficient buffer in all directions to protect the LS from the surrounding
rock radioactivity. The water pool is equipped with 2,400 20-inch PMTs,
mounted on the Main Structure, acting as a Cherenkov detector for the cosmic
muons. The muon detection efficiency is expected to be 99.8%, similar to that
of the Daya Bay water Cherenkov detector. Compensation coils are installed on
the Main Structure to protect the PMTs from the Earth magnetic field.

On top of the water pool, a muon tracker will be installed to measure the muon
directions. Plastic scintillator strips decommissioned from the Target Tracker
of the OPERA experiment [177] will be reused as the JUNO Top Tracker (TT). The
TT is composed of 62 walls with a sensitive area of 6.7$\times$6.7 m2 each.
Radioactivity from the surrounding rock will induce very high noise rates in
the plastic scintillator strips. These accidental backgrounds will be
suppressed with a multi-layer design with at least three x-y layers. The TT
covers more than 25% of the area of the top surface of the water pool.

Given the gigantic size and unprecedented energy resolution requirement,
construction of the JUNO detector is very challenging in mechanics, LS
radiopurity and transparency, PMT photon detection efficiency, reliability of
electronics, etc. In the following sections, we will describe the design and
R&D achievements.

###  3.2 Central Detector

####  3.2.1 Central Detector scheme and requirements

The 20 kton liquid scintillator detector, referred to as the Central Detector
(CD), is one of the most challenging parts of JUNO, especially in mechanics,
given its huge size. The LS will be contained in a spherical Acrylic Vessel
with an inner diameter of 35.4 m and a thickness of 120 mm. The Acrylic Vessel
will be supported by a spherical stainless steel shell structure with an inner
diameter of 40.1 m via 590 Connecting Bars. The shell structure sits on a
bearing consisting of 30 pairs of Supporting Legs, made of stainless steel
truss structures and rooted on the concrete floor of the Water Pool. The shell
structure and its bearing is called the Main Structure and is shown in Fig.
11. A total of 17,612 20-inch and 25,600 3-inch PMTs are installed on the Main
Structure and point inward to detect the scintillation light. Another 2,400
20-inch PMTs of the water Cherenkov detector are also installed on the Main
Structure and point outward to detect the water Cherenkov light. The CD and
the water Cherenkov detector are optically separated. The Main Structure also
provides support for the front-end electronics, cables, and the anti-
geomagnetic field coil. The whole structure must be stable and reliable for 30
years of the designed JUNO lifetime.

Figure 11: Scheme of the Central Detector (CD). The spherical Acrylic Vessel
is supported by a spherical stainless steel shell structure via 590 Connecting
Bars. The shell structure sits on a bearing consisting of 30 pairs of
Supporting Legs made of stainless steel truss structures.

To satisfy the requirements for the reactor and solar neutrino program, the CD
must be made of selected radiopure materials. The surfaces of the CD
components, including the Acrylic Vessel, pipes, pumps, and valves, are
required to be extra clean and LS/water-compatible. Besides, the air-tightness
of the CD sealing system must be very high to prevent Radon leakage from the
air.

####  3.2.2 The LS container: Acrylic Vessel

The Acrylic Vessel will be made by bulk polymerization of 265 pieces of
spherical acrylic panels. The acrylic recipe is carefully tuned with extensive
R&D, which does not include plasticizer and anti-UV material. It increases the
light transparency significantly and improves the anti-aging and creep
resistance performance. A dedicated acrylic panel production line is
established specially for JUNO to reduce the radioactive background and
increase transparency. It features fully closed pipelines and additional
filters for the liquid Methyl Methacrylate (MMA), the acrylic monomer. A
cleanroom of class 10000 and a high-purity water system are used to ensure the
mold’s cleanliness for the acrylic panel and the process of filling MMA into
the mold. The radiopurity of acrylic reaches a level of $<0.5$ ppt for uranium
and thorium. The process of unmolding and thermoforming at high temperatures
is optimized to increase the spherical acrylic panels’ transparency to $>96$%
in the water.

To ensure a lifetime of 30 years, the maximal pressure on the acrylic is
required to be less than 3.5 MPa in the long term and 7 MPa in the short term,
based on extensive dedicated experiments at IHEP. The Acrylic Vessel is
supported by connection structures linked to the Main Structure. The
supporting nodes on the Acrylic Vessel are critical to controlling the stress.
The design is optimized with many prototype tests. The ultimate bearing
capacity of the supporting nodes can reach 100 t, which is more than 6 times
the designed load.

A new bonding method has been developed for JUNO to reduce the onsite
construction time of the Acrylic Vessel. All the acrylic panels in the same
layer will be bonded simultaneously. This technique has been verified with
many tests and has been successfully applied in other projects.

####  3.2.3 Main Structure

The stainless steel Main Structure is designed as a shell structure of 30
longitudinal H-beams and 23 latitudinal H-beams. Five in-plane supports are
uniformly arranged along the longitudinal direction to improve the structure’s
torsional stiffness and stability. The shell structure is supported by 30 pair
Supporting Legs and 60 base plates, rooted on the concrete floor of the Water
Pool. By optimizing the planar truss column’s geometry, the reaction force of
the inner and outer rings of bearings is nearly uniform. Under the working
condition of about 3000 ton buoyancy applied on the Acrylic Vessel, the
reaction force of each support base is kept below 60 tons. The Main Structure
is designed to be safe for earthquakes of a horizontal acceleration of 46.5
Gal, corresponding to a 25% exceedance probability in 50 years.

A total of 590 Connecting Bars are designed to connect the Main Structure and
the Acrylic Vessel. At the Acrylic Vessel side, the Connecting Bars end in
hinged connections. At the Main Structure side, 220 Connecting Bars between
the 1st and 9th latitudinal layers end in disc springs to ensure that the
Acrylic Vessel node’s stress is less than 3.5 MPa with the buoyancy applied.
Another 370 Connecting Bars from the 10th to 23rd layers use the rigid
connection to control the structure’s overall stability.

Pre-assembly of the supporting structure and part of the Main Structure have
been tested in the factory. Key technologies for manufacturing and
installation of the Main Structure have been developed, including the
friction-type high-strength connecting technology, the anti-corrosion coating
scheme for the disc spring, the monitoring system of the axial force and
temperature of the Connecting Bars, etc.

####  3.2.4 The Filling, Overflow, and Circulation (FOC) system

The FOC system has one storage and two overflow tanks with a volume of 50 m3
each. It also includes a nitrogen flushing system, the piping connected to the
chimney of the CD, and a control platform, as shown in Fig. 12. It has three
primary functions: 1) filling pure water into the Acrylic Vessel synchronously
with the filling of the Water Pool, then replacing water with LS during the
6-month LS production period; 2) stabilizing the LS level in the Acrylic
Vessel within 20 cm when the liquid temperature varies within $21\pm
1.4^{\circ}$C during operation; 3) circulating the LS in the detector through
the underground LS purification systems (See Fig. 15) for the online
purification.

Figure 12: Scheme of the Filling, Overflow, and Circulation (FOC) system.

###  3.3 Veto detector

The experiment will be equipped with two veto systems providing both an
efficient background reduction towards the environmental radioactivity and the
residual cosmic muon flux crossing the detector.

####  3.3.1 Water Cherenkov Detector (WCD)

As shown in Fig. 3, the Water Cherenkov Detector (WCD) is a cylinder of 43.5 m
in diameter and 44 m in height, which is filled with 35 ktons of ultrapure
water. The Cherenkov light produced by muons passing through the volume is
detected by 2400 20-inch Microchannel Plate Photomultipliers (MCP PMTs)
installed on the outer surface of the stainless steel Shell Structure of the
CD. Tyvek reflective foils provide a coating for the pool walls and stainless
steel support structure to increase light collection efficiency [178]. The
passive water shielding significantly reduces the gamma background in the CD
due to radioactivity in the rocks.

The performance of the WCD requires stable working conditions over the future
duration of the experiment. A water system will provide and monitor the ultra-
high purity of the water and will also ensure temperature uniformity over the
whole volume.

The water system includes two plants for water production and circulation, one
on the surface and one underground. The surface plant is the production stage
with a rate of 100 tons/hour. High-purity water will be transported by
stainless steel pipes through the 1300 m long slope tunnel to the underground
plant for further purification in circulation in the WCD.

The detector consists of materials with different thermal properties, for
example, the Acrylic Vessel and the stainless steel Main Structure. The water
temperature around the Acrylic Vessel has to be stabilized at (21$\pm$1 ℃) to
maintain the detector’s mechanical stability. The heat from the electronics
box for PMTs ($\sim$260 kW) on the stainless steel support structure needs to
be taken away by water circulation. The requirement to control the temperature
at such a precision level in a vast volume is very challenging. The
underground water plant was optimized for high precision temperature control
in the water circulation, which occurs separately in two volumes, an inner
part in the Central Detector and an outer part in the WCD volume. The relevant
parameters are optimized, including the temperature and flow rate of inlets,
the size and the number of holes on the water distributor, the flow rate ratio
between the top and bottom inlets, etc. The water inlets are located on the
top and bottom of the water pool, whereas the outlet is on the equator. Water
distribution systems are designed for both the top and bottom inlets to make
the water flow uniform. The temperature stability requirement has been
achieved for the entire surface of the Acrylic Vessel with extensive
simulations.

The radon concentration in tap water is about 10 Bq/m3 [179]. It must be
reduced to $<0.2$ Bq/m3 for reactor antineutrino oscillation studies. A factor
of 1000 ($\sim$10 mBq/m3) reduction is possible with our R&D, ideal for other
low energy physics. A radon removal system with a 3-stage degassing membrane
system is integrated into the water system. In a prototype with a 3-stage
degassing membrane, the radon concentration in water has been controlled to
$<50$ mBq/m3 [180, 181]. The latest results show that the radon concentration
could be controlled at $\sim 10$ mBq/m3 in the prototype water. The radon
removal efficiency of the first stage reaches 90%. The second and third stages
were found to be less efficient due to a lower gas content in water.
Therefore, a micro-bubble system fed with high-purity nitrogen is added to the
second and third stages and improves the removal efficiency to above 90%. For
JUNO water system, we adopt the multi-stage degassing membrane to reduce radon
in the water. Simultaneously, it should have strict requirements on the
cleanliness of the detector installation to prevent excessive dust from
accumulating inside the detector to introduce radioactive contamination(such
as radium).

An HDPE film will be used as a liner to cover the pool walls and prevent
radon’s incursion from external rocks. According to the emanation measurements
on several rock samples, including those from JUNO and Daya Bay sites, an HPDE
film thickness of 5 mm is required to keep the radon concentration in the
water below 10 mBq/m3. A 5 mm thick HDPE film will be used. The gaps between
adjacent pieces of HDPE film will be closed by HDPE strips attached by hot-
melt welding. During the HDPE installation, a network of electrodes will be
buried in the cement to perform a leakage test after the liner installation.

The pool will be covered with a black rubber sheet that is sealed by a gas-
tight zipper. This type of cover has been used in the Daya Bay experiment and
shows excellent sealing performances. Due to the chimney interface in the
center of the pool, the cover will be divided into two semicircles and sealed
by gas-tight zippers withstanding a maximum pressure of 100 kPa. Gas tightness
is very difficult to maintain due to the large surface of more than 1000 m2.
Therefore, the cover is placed 1 meter higher than the water surface. The
volume in between will be filled with high-purity nitrogen with a positive
pressure of a few hundred Pa to prevent radon contamination from outside.

Since the Top Tracker on top of the pool has a good efficiency in detecting
muons, the WCD is optimized to have 1045 and 1355 PMTs on the upper and lower
hemisphere of the Main Structure, respectively. As such, the upper
hemisphere’s effective detection efficiency plus the Top Tracker equals that
of the lower hemisphere. The WCD PMTs are grouped into ten zones: 5 in the top
and 5 in the bottom hemisphere. A local trigger with a lower threshold is used
instead of the global trigger. If adopting global trigger, the muon detection
efficiency is anticipated to be 98% with a trigger threshold of 54 PMTs. The
local triggers include two cases. One case is the fired PMT number in one zone
above the threshold ($\sim$19 PMTs). Another is the fired PMTs number of two
adjacent zones simultaneously above a lower threshold ($\sim$13 PMTs). The
detection efficiency can reach 99.5% while keeping the noise at an acceptable
level.

The 20-inch PMTs in the Central Detector and the Water Cherenkov Detector may
be affected by the geomagnetic field, losing up to 60% detection efficiency. A
set of 32 circular coils surrounding the detector is designed to compensate
for the geomatic field. The residual magnetic field will be less than 0.05 G
in the Central Detector PMT region and lower than 0.1 G in the WCD PMT region,
satisfying the experimental requirements.

The muon flux in the experimental hall is 0.004 Hz/m2. The average muon energy
is 207 GeV. Based on the JUNO detector simulation, the average track length of
muons going through the pool is about 13 m. The muon tagging efficiency can
achieve 99.5%. Most of the un-tagged muons have short tracks ($<$0.5 meter)
and are far from the Central Detector, making little contribution to the fast
neutron background. The fast neutron background from muons spallation can be
controlled at a rate of about 0.1 event/day.

####  3.3.2 Top Tracker (TT)

The Top Tracker will be located above the WCD as shown in Fig. 13. As
discussed in Ref. [2], the JUNO Top Tracker (TT) will use existing
scintillating strips from OPERA’s Target Tracker [177]. Since the dismantling
of the OPERA detector (LNGS, Gran Sasso, Italy), the quality of the TT modules
has been monitored. We do not observe any extra deterioration of the plastic
scintillator properties from the current aging monitoring. At least $4.9\pm
1.5$ p.e. are observed for muons crossing the middle (most disfavored
position) of the plastic scintillator strips. The expected overall efficiency
of a TT module, as extrapolated from the previous experience, is at the level
of $(98.0\pm 0.5)\%$ for a 1 p.e. threshold. This number is due to the
scintillator properties, the threshold, and the geometry of the TT modules. It
has to be noted that in the experiment, a 1/3 p.e. threshold will be used.

20 m47 m3 m6.7 m6.7 mTT wall

Figure 13: The JUNO Top Tracker.

As shown in Fig. 13, “walls” of plastic scintillator, called TT walls, will be
placed in horizontal layers on top of the JUNO Central Detector rather than
vertically as it was done in OPERA. Given the TT walls are flexible, a
supporting structure was developed to limit the sag to less than 6 mm.

The TT walls will be distributed on 3 horizontal layers, separated by 1.5 m,
on a ${3\times 7}$ horizontal grid, as shown in Fig. 13. There is a 15 cm
overlap between adjacent walls of the same layer to avoid dead zones in the
detector. The 3 walls in the center of the TT are moved up to leave enough
space for the calibration house and the Central Detector chimney. To fit in
the available space below the cranes, these walls are also closer, with a
separation of only 23 cm.

With this geometry, about 1/3 of all atmospheric muons passing through the CD
will also cross 3 layers of the TT so that they can be tracked. In these
cases, the TT can be used as a veto, but for the remaining muons crossing the
CD, the veto strategy depends only on the WCD and CD. One notable region where
the TT veto is particularly effective is for atmospheric muons entering the
detector through the chimney region, which is well covered by the TT, and
might present difficulties for the other subsystems in some classes of events.

Each TT wall is made using a total of 512 strips of plastic scintillator.
Every 64 plastic scintillator strips, with dimensions ${6.7~{}\textrm{m}\times
2.6~{}\textrm{cm}\times 1.1~{}\textrm{cm}}$, are grouped in a TT module.
Wavelength-shifting fibers are placed in every strip, as shown on the left
part of Fig. 14. These fibers are read from both sides by 64-channel multi-
anode photomultipliers placed at each end of the TT modules, with every fiber
directly in front of a channel of the multi-anode photomultiplier. A group of
4 TT modules are placed side by side to form a ${6.7\times 6.7}$ m2 square,
forming a TT plane. Two TT planes are placed on top of each other, rotated by
90∘ to form a TT wall, as shown in the central part of Fig. 14.

6.7 m

6.7 m

2.6 cm

1.1 cm

CBROBFEBPMTRJ-45 cable: ROB – CBOptical link to

1.7 m

2.6 cm64 strips/moduleDAQ and GTBplastic scintillator stripTT module

Figure 14: Schematics of a TT wall, with detail of a plastic scintillator
strip (left), and photos of the TT wall electronics (right). Dimensions of a
TT wall, a TT module (one of the modules is shown in blue) and a plastic
scintillator strip are also drawn.

The natural radioactivity of the rock in the JUNO cavern is expected to create
a ${\sim 50}$ kHz per PMT background rate in the TT, based on the 238U, 232Th,
and 40K concentrations found in the rock prospected from the cavern site,
which are two orders of magnitude higher than in the LNGS underground
laboratory. This rate per PMT corresponds to a ${\sim 2}$ MHz full detector
rate, which is to be compared with the expected ${\sim 3}$ Hz of atmospheric
muons passing through the TT. The new TT readout electronics is designed to
work under these conditions, and remove quickly large fractions of events
produced by this background.

The TT readout electronics chain is composed of 4 different boards. The Front-
End Board (FEB) and the Read-Out Board (ROB) are located next to the multi-
anode 64 channels PMT, at each end of the modules. These cards perform the PMT
digitization, the slow control and provide the high-voltage (HV) supply.
Digitization of any charge detected with the ROB takes about 7 $\mu$s. In the
center of every wall, grouping 16 FEB – ROB pairs is placed a Concentrator
Board (CB), as shown in Fig. 14. The CB does the timestamping of the hits
observed and is responsible for the first level trigger (L1) by requiring 2D
coincidences in the same wall. If the CB does not find a coincidence, it can
reset the acquisition being performed on the FEB – ROB pairs within 300 ns in
order to reduce the acquisition dead time. Finally, the Global Trigger Board
(GTB) will be placed near the center of the detector to provide a global (L2)
trigger to the TT by requiring at least 3 aligned walls on different layers to
have an L1 trigger before the event is accepted. As is the case of the CB, the
GTB can also reset the acquisition in case no coincidence is found. However,
this feedback is provided within about 1 $\mu$s in this case.

The L1 trigger produced by the CB reduces the detected rate by about one order
of magnitude, while the L2 trigger produced by the GTB further reduces the
rate by two orders of magnitude, reaching a ${\sim 2}$ kHz detection rate for
radioactive events. As discussed above, both triggers reset the acquisition in
the FEB – ROB after about 300 ns and 1 $\mu$s, respectively, to reduce the
detector’s dead time in case that no coincidence is found. While this rate is
still about 3 orders of magnitude larger than the expected atmospheric $\mu$
rate crossing the TT, the TT off-line reconstruction is able to further reject
this background by requiring the alignment of the crossing points between
individual strips, rather than of the alignment of walls, which is done by the
L2 trigger.

Thanks to the small granularity of the TT of ${2.6\times 2.6}$ cm2, using this
trigger logic and the TT reconstruction, the efficiency for the TT to
reconstruct atmospheric muons is of about 93% and their median angular
resolution is of 0.20∘. By also requiring a corresponding signal to be seen in
the CD or WCD, the TT provides a well reconstructed muon sample with purity
$>99$% that can be used to calibrate and to tune reconstruction algorithms in
the CD and WCD.

Well-reconstructed muons from the TT will also be used to measure the
distribution of the distance between the muon and the cosmogenic isotopes
produced in the CD, which is a key component in deciding the size of the muon
veto cylinder used in the analysis. For these well-tagged isotopes (9Li and
8He), another important parameter, the energy distribution, can also be
measured and used in the JUNO physics analysis. In addition to that, the TT
should contribute to the estimation of the fast-n rate experimentally, thus
avoiding uncertainties in the simulation of neutron production, propagation
and interaction in the detector.

###  3.4 Liquid scintillator

The preparation of the liquid scintillator (LS) requires high quality of all
the chemicals to satisfy the strict requirements of JUNO. There are three
components in the JUNO LS recipe: Linear Alkyl Benzene (LAB),
2,5-diphenyloxazole (PPO), and 1,4-bis(2-methylstyryl)benzene (bis-MSB). The
optimal LS composition was determined to be the purified solvent LAB with 2.5
g/L PPO and 3 mg/L bis-MSB [176]. More than five years of R&D efforts were
devoted to raw material procurement and the design of the purification plants
in order to produce the best possible LS in terms of optical and radio-purity
properties. For the raw materials, one LAB factory succeeded in providing
high-quality LAB with a long attenuation length at 430 nm. This company
already produced 20 tons special LAB for JUNO LS pilot production, achieving
an attenuation length of around 25 m at 430 nm. The LAB will be transported by
dedicatedly cleaned ISO-tanks with nitrogen covering, before being stored in a
5000 ton stainless steel tank and purged by nitrogen onsite. Similar long-term
cooperation was established with one PPO supplier, which was able to provide
purified PPO with residual contamination in 238U and 232Th around 1 ppt after
a great effort.

The requirement on the U/Th radiopurity of the LS is $1\times 10^{-15}$ g/g
for the reactor neutrino studies and $1\times 10^{-17}$ g/g for the solar
neutrino studies. It is difficult to quantitatively demonstrate that the LS
could be purified to such a low level with our purification systems, until we
can use the data of the JUNO detector. Therefore, the minimum requirement is
determined to be $1\times 10^{-15}$ g/g upon filling, with a target
radiopurity of $<1\times 10^{-16}$ g/g. The LS will be further purified
online, with the water extraction and gas stripping systems, to reach the goal
of $1\times 10^{-17}$ g/g.

####  3.4.1 Purification systems

To further improve optical and radio-purity properties of LS, a combined
system of purification plants has been designed as shown in Fig. 15. The
plants will be operated in a sequence following a strategy optimized with a
successful test done at Daya Bay using pilot plants [182]. In the JUNO surface
area, raw LAB will pass through the alumina columns and the distillation
plant, before being mixed with PPO and bis-MSB. The resulting LS will be sent
underground through an electro-polished pipe for the last two purification
stages, water extraction and stripping plants. After the final quality check
in the Online Scintillator Internal Radioactivity Investigation System
(OSIRIS) detector, the LS will be sent to the FOC system.

Figure 15: Flowchart of the liquid scintillator processing system.

In order to reduce radon emanation and diffusion, the internal surface
roughness of all the equipments is kept below 0.4 $\mu$m with mechanical
polishing followed by electrochemical polishing. Furthermore, all joints are
realized with double O-ring flanges with internal nitrogen purging and a
tested overall leakage rate less than 10-6 mbar$\cdot$l/s. In addition,
besides normal nitrogen, a high purity nitrogen system is designed for
blanketing and running of all the plants. The system consists of two Low
Temperature Absorber (LTA) columns filled with ultralow radioactive background
activated carbon. One LTA column can continuously work for more than 7 days at
50 Nm3/h flux rate before its activated carbon needs to be regenerated. The
radon content of the high purity nitrogen can be less than 10 $\mu$Bq/m3. The
two-stage filters (220 nm and 50 nm) are used in each sub-system to remove
small particulates in LS and to reduce the 238U and 232Th radioactive
background.

##### Alumina column system

The LAB will be purified by the alumina (Al2O3) columns to improve
transparency. Based on the pilot plant experiments, we determined the final
design of the alumina filtration system. The results show that the system can
significantly improve the attenuation length of LAB without apparent increase
of the radioactive background (below the measurement sensitivity of $\sim
10^{-15}$ g/g). The system consists of eight filtration columns, two alumina
filling tanks, and three buffer tanks. The height of the column is 2.6 m, and
the diameter is 60 cm. The column can work at a height-to-diameter ratio of
3:1 or 4:1 to keep a high purification efficiency. The alumina will be packed
in vacuum aluminum film bags for protection from radon. The alumina will be
filled in the filling tank via a glove box to minimize possible radon
contamination.

##### Distillation system

Distillation in a partial vacuum (5 mbar) is used in the second stage of the
purification process to remove the heaviest radio-impurities (238U, 232Th and
40K) from the LAB and to further improve its optical property in terms of
absorbance and attenuation length. After a preheating up to 190∘C inside a
counter-current heat exchanger (heat recovery), the LAB is delivered to the
distillation column (7 m height and 2 m wide with 6 sieve trays) where the
purification process is carried out with a counter-current flow of gaseous LAB
produced inside a tube-bundle hot oil reboiler.

The gas stream is collected in the top of the column and liquefied in a
condenser cooled with chilled water at 35∘C from a water-cooling tower. The
purified LAB flow is partially sent back on the top of the column as reflux to
increase the purification efficiency (up to 40%). The distillation plant is
designed to operate with a nominal discharge flow from the column bottom of
1–2% of the input stream, in order to get a better compromise between the
product purity and reasonable throughput.

The LAB distillation technique was proved effective in terms of radio-purity
and optical transparency in the pilot plant experiments performed at Daya Bay
[182].

##### Mixing system

The mixing system is located in the surface LS hall. It is responsible for
dissolving PPO and bis-MSB into LAB to form LS master solution batch by batch.
Acid/water washing is also performed to the master solution to purify PPO and
bis-MSB at this stage. Possible master solution distillation techniques to
remove 238U/232Th from PPO are under study. Finally, the master solution will
be diluted with LAB to form LS and transported to underground facilities.
Besides double O-ring flanges, glove box feeding and magnetic fluid seal are
used to ensure the good sealing performance of the system.

##### Water extraction system

The water extraction system is used to remove 238U and 232Th backgrounds from
the LS, especially impurities introduced by PPO and bis-MSB. The water
extraction system has been designed based on laboratory studies, pilot plant
experiments, and theoretical calculations. It is a turbine extraction tower of
13 meters high and 1 meter in diameter, with five theoretical stages. Water
and LS are fully mixed by turbine stirring. The LS flow rate is 7 m3/h, while
the water flow rate is adjustable within 1.4–2.3 m3/h. The purified LS is
first fed into a static separation tank before the final product tank.
Continuous nitrogen purge in the LS in the product tank ensures the water
content below 200 ppm. In order to reduce the interfacial fouling produced by
extraction, there is a sewage discharge port at the LS-water interface. The
water with ultra-low 238U/232Th and Rn concentration will be provided by an
Ultrapure Water (UPW) system, which is based on the highest UPW requirements
and technologies for the semiconductor industry. The 238U and 232Th of UPW
will be $<1\times 10^{-15}$ g/g, which corresponds to $<1\times 10^{-16}$ g/g
LS radiopurity given the water/LAB partition coefficient being 10, and could
be upgraded in the future.

##### Stripping system

The stripping plant is the final stage of the purification procedure. The main
purpose of the plant is the removal of radioactive gases and gaseous
impurities from the liquid scintillator phase, using a gaseous stream of
nitrogen and/or superheated steam in counter-current flow mode. The
purification process is performed inside a 9-meters-high stripping column,
filled with AISI 316 L metal pall rings. LAB entering the plant is pumped at 7
m3/h by a magnetic driven pump through a set of input fine filters (50 nm pore
size) and heated up by tube-bundle heat exchangers up to 90∘C, to reduce the
LAB viscosity and increase the process efficiency. The stripping column is fed
at the top with pre-heated liquid LAB while at the bottom with an adjustable
mixture of ultra-pure nitrogen and UPW steam (30–60 kg/h). The stripping
process is performed in a partial vacuum, at about 250 mbar. During pilot
plant tests, carried out at the Daya Bay laboratory in 2018, this combination
of parameters allowed to achieve 95% purification efficiency in Rn removal
[182]. The full-size plant has been improved with a 50% higher stripping
column; therefore we expect better efficiency.

####  3.4.2 OSIRIS

The Online Scintillator Internal Radioactivity Investigation System (OSIRIS)
is a stand-alone detector to monitor the radiopurity of the LS while the JUNO
CD is filled and to confirm the proper operation of the purification plants.
The aim is to guarantee that the concentrations of 238U and 232Th in the LS do
not exceed the given limits of 10-15 g/g or 10-16 g/g for the IBD or solar
neutrino measurement, respectively. The measurement is based on observing the
easy-to-identify fast coincidence decays of 214Bi-214Po ($\tau\sim 164$
$\mu$s) and 212Bi-212Po ($\tau\sim 0.43$ $\mu$s) in $\sim 20$ m3 of LS, or
$\sim 17$ tons of LS. The decays indicate the presence of 238U and 232Th,
respectively. Based on identifying a handful of coincidence events per day,
OSIRIS can reach the IBD sensitivity level within a few days and the solar
level within 2–3 weeks.

Figure 16: Scheme of the OSIRIS detector.

OSIRIS is located in a bypass to the main LS line, permitting to sample a
relevant fraction ($\sim 1/6$) of the LS processed in the purification plants.
The detector design is illustrated in Fig. 16. It consists of a cylindrical
Acrylic Vessel (AV) with a height and diameter of 3 m holding the LS. The AV
is mounted inside a water-filled steel tank (height and diameter 9 m), which
provides 3 m.w.e. of shielding against external radiation. This is sufficient
to reduce accidental coincidences of external $\gamma$ events to a negligible
level. The main background to the measurement will be formed by the radon
contamination of the LS that stems from the emanation of components during the
LS purification process.

The scintillation light from the decay events is collected by 64 20-inch PMTs
(Hamamatsu R12860) that are mounted to a stainless-steel frame surrounding the
AV at a distance of 1.3 m facing towards the AV. In addition, 12 20-inch PMTs
are directed outwards and act as a muon veto detector. These two subsets of
PMTs will be optically separated by a PET foil installed to the stainless-
steel frame. The expected photoelectron yield is $\sim$275 p.e. at 1 MeV,
corresponding to an energy resolution of $\sim 6$% at 1 MeV for the inner
detector. The outer PMTs provide a muon tagging efficiency of $>85$%.

One design feature of OSIRIS is that the data acquisition electronics is
mounted directly to the back of each PMT, a novel concept called intelligent
PMT (iPMT). Each iPMT acts as an individual detector with a large dynamic
range (1–103 p.e.). The analog signal of the PMT is continuously sampled
inside the iPMT using a highly integrated receiver chip (VULCAN). In case the
trigger threshold of an individual PMT is exceeded, the waveform is sent via
the network to the DAQ system. There, the data of all PMTs is time-sorted, and
a software trigger extracts the physical events, enabling a very flexible
implementation.

The OSIRIS detector holds two calibration systems. The source-insertion system
re-uses one of the Automatic Calibration Units (ACU) of the Daya Bay
experiment. It will be mounted at an off-axis location on the top of the
OSIRIS tank and is connected to the AV via an LS-filled pipe. The system is
equipped with two capsules containing weak radioactive $\gamma$-sources and
one LED to enable energy and position calibration. A second system feeds the
pulses of an external picosecond laser system via optical fibres to an array
of diffusers mounted to the steel frame holding the PMTs. It will be used for
precise timing and single p.e. charge calibration.