The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid scintillator multi-purpose underground detector, under construction near the Chinese city of Jiangmen, with data collection expected to start in 2021. The main goal of the experiment is the neutrino mass hierarchy determination, with more than three sigma significance, and the high-precision neutrino oscillation parameter measurements, detecting electron anti-neutrinos emitted from two nearby (baseline of about 53 km) nuclear power plants. Besides, the unprecedented liquid scintillator-type detector performance in target mass, energy resolution, energy calibration precision, and low-energy threshold features a rich physics program for the detection of low-energy astrophysical neutrinos, such as galactic core-collapse supernova neutrinos, solar neutrinos, and geo-neutrinos

L. Miramonti

Universe

Lino Miramonti

Multidisciplinary Digital Publishing Institute

Neutrino Physics and Astrophysics with the JUNO Detector

AIR Universita degli studi di Milano

universe
Conference Report
Neutrino Physics and Astrophysics with the
JUNO Detector
Lino Miramonti and on behalf of the JUNO Collaboration
Dipartimento di Fisica, Universitá degli Studi di Milano and I.N.F.N. Sezione di Milano, Via Celoria 16,
20133 Milano, Italy; lino.miramonti@mi.infn.it; Tel.: +39-025-031-304
Received: 12 October 2018; Accepted: 13 November 2018; Published: 16 November 2018 
Abstract: The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid scintillator
multi-purpose underground detector, under construction near the Chinese city of Jiangmen, with data
collection expected to start in 2021. The main goal of the experiment is the neutrino mass hierarchy
determination, with more than three sigma significance, and the high-precision neutrino oscillation
parameter measurements, detecting electron anti-neutrinos emitted from two nearby (baseline of
about 53 km) nuclear power plants. Besides, the unprecedented liquid scintillator-type detector
performance in target mass, energy resolution, energy calibration precision, and low-energy threshold
features a rich physics program for the detection of low-energy astrophysical neutrinos, such as
galactic core-collapse supernova neutrinos, solar neutrinos, and geo-neutrinos.
Keywords: JUNO; liquid scintillator; neutrino mass hierarchy
1. Introduction, How to Infer the Mass Hierarchy
Among the remaining undetermined fundamental aspects in the leptonic sector of the standard
model, there is the neutrino mass hierarchy (MH) determination (i.e., whether the ν3 neutrino mass
eigenstate is heavier or lighter than the ν1 and ν2 mass eigenstates). The mass hierarchy discrimination
will have an impact in the quest for the nature of the neutrino (Dirac or Majorana) [1]. The two possible
scenarios, still compatible with the huge amount of data from different classes of neutrino experiments,
usually denoted as normal and inverse hierarchy (NH and IH), are represented in Figure 1. Denoting
the squared mass eigenvalues differences as ∆m2ij = m
2
i −m2j , in the normal hierarchy case the third
neutrino mass eigenvalue would be the biggest one and |∆m231| = |∆m232| + ∆m221. Instead, in the
inverted hierarchy case, m3 would be the smallest eigenvalue and the relation would be |∆m231| =
|∆m232| − ∆m221. According to the most recent global analyses, the squared mass differences are1:
∆m221 = (7.37± 0.17)× 10−5eV2; (1)
|∆m232| = (2.52± 0.04)× 10−3eV2 . (2)
1 The first result is obtained mainly from solar neutrino and from KamLAND experiments. The second result is obtained
mainly from atmospheric and by long baseline accelerator experiments. This second value would correspond to |∆m231| in
the case of inverted hierarchy.
Universe 2018, 4, 126; doi:10.3390/universe4110126 www.mdpi.com/journal/universe
Universe 2018, 4, 126 2 of 7
Normal
m12
solar: 7.5×10-5 eV2
m22
atomospheric:
2.4×10-3 eV2
m32
Inverted
m12
atomospheric:
2.4×10-3 eV2
m22
solar: 7.5×10-5 eV2
m32
νe νµ ντ
Figure 1. Neutrino mass eigenstate flavor composition and mass pattern in the two cases of normal
(left) and inverted (right) hierarchies. Taken from Ref. [2].
The relatively high value of the θ13 angle, (θ13 = 8.46◦+0.15
◦
−0.15◦ , at 1σ for the normal hierarchy [3])
makes possible the realization of the idea [4,5] to extract the mass hierarchy from the study of
hierarchy-dependent oscillation probability corrections, which are proportional tosin2 θ13.
We can write the ν¯e survival probability in the form2:
Pee = 1− cos4θ13 sin22θ12 sin2
∆m221L
4E
− sin22θ13
(
cos2θ12 sin2
∆m231L
4E
+ sin2θ12 sin2
∆m232L
4E
)
= 1− cos4θ13 sin22θ12 sin2
∆m221L
4E
− PMH (3)
The last term of Equation (3), indicated as PMH , represents the mass hierarchy-dependent
contribution to the oscillation probability. It can be rewritten as:
PMH =
1
2
sin22θ13
1−
√
1− sin22θ12sin2(
∆m221L
4E
) cos
(
2
∣∣∣∣∆m2eeL4E
∣∣∣∣± φ)
 (4)
where ∆ m2ee represents the combination ∆ m2ee = (cos2θ12∆m231 + sin
2 θ12∆m232). The new quantity φ is defined in
such a way that sin φ and cos φ are combinations of the mass and mixing parameters in the 1–2 sector3. The ± sign
in the last term of Equation (4) is decided by the MH, with plus sign for the normal one and minus sign for
the inverted one. Hence, the survival (or oscillation) probability and consequently the observed spectrum are
characterized by the presence of rapidly oscillating terms. Phases are in opposition for the two MH cases (NH and
IH), superimposed to the general oscillation pattern valid for both hierarchies.
In order to discriminate between the two possible hierarchies, we need to determine the ν energy spectrum
with a very high energy resolution. The experimental signature of the reactor νe is given by the inverse beta
decay process νe + p→ e+ + n. The resulting signal is given by visible energy from the positron energy loss and
annihilation, plus delayed light, at a fixed 2.2 MeV energy, from the neutron capture on protons.
Figure 2 shows the spectrum in the non-oscillating case (dashed line) and the expected spectrum for a detector
with a baseline of about 50 km, as in the JUNO experiment case. The hierarchy-dependent corrections are in phase
opposition—the blue line represents the NH and the red line the IH.
2 Pee = 1− P12 − P13 − P32 with P’s weights as 40:2:1.
3 For details of the calculation see Ref. [2].
Universe 2018, 4, 126 3 of 7
Figure 2. The relative shape difference of the reactor antineutrino flux for different neutrino mass
hierarchies (MHs) [2]. The figure represents the product of the neutrino flux times the interaction cross
section times the survival probability.
2. JUNO Detector and Capabilities
The JUNO (Jiangmen Underground Neutrino Observatory) detector will be built at a shallow depth, with an
average overburden of about 700 m, close to Jiangmen City, China, at a 53 km distance from the Yangjiang and
Taishan nuclear power plants. The foreseen total power produced by the two facilities is about 26.6 GW.
The core of the detector is composed of 20 kton of liquid scintillator (LS), contained in an acrylic sphere
(12 cm thick, with an inner diameter of 35.4 m), kept in position by a stainless steel truss. This central detector
is placed inside an active water pool that will act as a Cherenkov muon veto and will reduce gamma rays and
neutrons coming from the rock. On top of the water pool there will be an external veto detector composed of
plastic scintillators.
The LS is composed of LAB (linear alkyl benzene) as solvent, with PPO (2.5-diphenyloxazole) as fluor and
bis-MSB, a wavelength shifter.
In order to attain the desired energy resolution to disentangle the two mass hierarchies, the JUNO
Collaboration paid particular attention to the photomultiplier system, which consists of 17,000 large PMTs,
with a diameter of 20 inches, interspersed with 25,000 smaller ones of 3 inch diameter. This large number of PMTs
will allow powerful event reconstruction and will offer an internal cross-check, which should help in reducing
the non-stochastic uncertainty and improving the high-precision oscillation parameter measurement. The total
coverage is about 80%, corresponding to about 1200 photo-electrons/MeV.
To analyze the spectrum, distinguish the position of variation in the flux oscillation, and extract the desired
mass hierarchy information, the total energy resolution must be σE√
E
≤ 3%.
3. Mass Hierarchy and Neutrino Physics
The main goal of JUNO is to establish the neutrino mass hierarchy by analyzing the spectrum of the
antineutrino inverse β decay on proton. To obtain this result, the experimental spectrum will be compared with
the theoretical one, which is a function of the oscillation parameters and of the mass hierarchy.
This discrimination is obtained by comparing the χ2 values corresponding to the two different
hierarchies’ best fit points.
The term ∆χ2MH =
∣∣χ2MIN(NH)− χ2MIN(IH)∣∣ is a measure of the mass hierarchy sensitivity, and represents
the discrimination power of JUNO [2].
In addition to the mass hierarchy determination, JUNO will perform other measurements and analyses.
Thanks to the very high statistics and the excellent energy resolution JUNO will provide a significant improvement
in the precision measurement of the three masses and mixing parameters (see Table 1) [3].
Universe 2018, 4, 126 4 of 7
Table 1. Comparison of the present accuracy and the JUNO (Jiangmen Underground Neutrino
Observatory) potentialities for three oscillation parameters.
Oscillation Parameter Current Accuracy Dominant Experiment (s) JUNO Potentialities(Global 1σ)
∆m221 2.3% KamLAND 0.59%
∆m2 =
∣∣∣m23 − 12 (m21 +m22)∣∣∣ 1.6% MINOS, T2K 0.44%
sin2 θ12 ∼ 4− 6% SNO 0.67%
4. Neutrino Astrophysics
4.1. Supernova Neutrinos
The rich JUNO programme also covers neutrino astrophysics. A supernova (SN) explosion causes the
emission of a huge amount of energy in the form of neutrinos over a long time period. JUNO can perform studies
of supernovae, both with the direct measurement of the neutrino burst produced by a nearby SN collapse and
with the study of the diffuse supernova background (DSNB).
Analyzing the three different stages in which the SN explosion process can be divided (i.e., the very fast
initial neutrino burst, the accretion phase, and the cooling phase), it is possible to infer many parameters of interest
for astrophysics and for elementary particle physics (see Figure 3).
These three phases have different emission spectra, and thanks to this difference it is possible to obtain
complementary information on the neutrino (antineutrino) flavor composition and luminosity and to probe SN
models in term of time evolution, energy spectra, and flavor mixing.
Figure 3. Three phases of neutrino emission from a core-collapse supernova (SN), from left to right:
(left) Infall, bounce, and initial shock-wave propagation, including prompt νe burst. (middle) Accretion
phase with significant flavor differences of fluxes and spectra and time variations of the signal.
(right) Cooling of the newly formed neutron star, with only small flavor differences between fluxes and
spectra. νx collectively stands for neutrinos and antineutrinos of all three flavors. Figure from Ref. [2].
Table 2 lists the main interaction channels and the number of expected events in JUNO, for a 10 kpc
distant SN.
Table 2. Expected number of events in JUNO for the main channels for neutrinos produced by a SN at
a distance of 10 kpc. Taken from Ref. [2].
Process Type Events〈Eν〉 = 14 MeV
ν¯e + p→ e + + n CC 5.0 × 103
ν+ p→ ν+ p NC 1.2× 103
ν+ e− → ν+ e− ES 3.6× 102
ν+12 C → ν+12 C∗ NC 3.2× 102
νe +12 C → e− +12 N CC 0.9× 102
ν¯e +12 C → e+ +12 B CC 1.1× 102
Universe 2018, 4, 126 5 of 7
4.2. Solar Neutrinos
Solar neutrinos can also be studied with the JUNO detector. Despite the shallow site, the JUNO detector takes
advantage of its large LS mass (which guarantees a very high statistics), and of its very good energy resolution.
For these studies, a key parameter is the radiopurity level of the LS, in order to disentangle the signal from
the different natural background sources. In particular, the radioactive isotopes 238U, 232Th, 40K, 210Bi, and 85Kr
must be considered. Table 3 shows the background rate constraints required to conduct low-energy solar neutrino
measurements, as well as the estimated solar neutrino signal rates at JUNO.
Table 3. The requirements ofsingles background rates for conducting low-energy solar neutrino
measurements and the estimated solar neutrino signal rates at JUNO. Taken from Ref. [2].
Internal Radiopurity Requirements
Baseline Ideal
210Pb 5× 10−24 (g/g) 1× 10−24 (g/g)
85Kr 500 (counts/day/kton) 100 (counts/day/kton)
238U 1× 10−16 (g/g) 1× 10−17 (g/g)
232Th 1× 10−16 (g/g) 1× 10−17 (g/g)
40K 1× 10−17 (g/g) 1× 10−18 (g/g)
14C 1× 10−17 (g/g) 1× 10−18 (g/g)
cosmogenic Background Rates (counts/day/kton)
11C 1860
10C 35
Solar Neutrino Signal Rates (counts/day/kton)
pp ν 1378
7Be ν 517
pep ν 28
8B ν 4.5
13N/15O/17F ν 7.5/5.4/0.1
One of the major concerns in solar neutrino physics is the so called “solar metallicity problem” (SMP),
about the content of metals in the core of the Sun [6]. These different possible versions of low Z vs. high Z of the
standard solar model affects the neutrino fluxes. An improvement in the accuracy of the determination of the flux
for 7Be and for 8B could help in solving the SMP.
As shown in the left panel of Figure 4, the experimental results were compatible with the predictions for
both the low Z and high Z models. This means that it is not possible to definitely solve this ambiguity only by
looking at 7Be and 8B fluxes. A breakthrough would come with the measurement of the CNO neutrino fluxes, as
shown in the right panel of Figure 4. This graph also indicates that a parallel improvement in the determination of
one of the two fluxes, measurable with the JUNO detector, would bring a complementary piece of information,
essential to discriminate the ambiguity between high-Z models and models predicting low Z metallicity with
modified opacity.
The possibility of observing 8B neutrinos is guaranteed in an experiment characterized by a high photon
yield, like JUNO, with a relatively low threshold and very good energy resolution. The main problem is due to the
cosmogenic background of long-lived spallation radioisotopes. The most troublesome for the νe elastic scattering
channel are 8Li, 16N, and particularly the 11Be, whose spectra cover almost all the energy regions of interest and
decay slowly (1 s).
Besides the standard νe elastic scattering, another channel for the 8B neutrino study is the charged current
interaction, mainly with 13C with an energy threshold of 2.2 MeV.
Another open issue in solar neutrino physics is the study of the transition region between the low-energy
vacuum oscillation and the higher energies, where the MSW mechanism dominates. It is important to test the
stability of the large mixing angle (LMA) oscillation solution, also considering additional contributions to the
“standard” oscillation pattern, due, for instance, to non-standard neutrino interactions (NSIs).
The high statistics and the excellent energy resolution of JUNO could perform a detailed study of the
spectrum in the transition region around 3–4 MeV.
Universe 2018, 4, 126 6 of 7
Figure 4. Theoretical predictions and experimental results for solar ν fluxes. The 8B flux is reported on
the x-axis. On the vertical axis, instead, one can see the fluxes of 7Be (left) and of 13N + 15O, from CNO
cycle (right). The 1σ experimental results correspond to black bars in the left graph and to shaded gray
vertical band in the right one. The 1σ theoretical allowed regions in high Z, low Z, and low Z with
modified opacity versions of the standard solar model (SSM) correspond, respectively, to black, red,
and blue ellipses. Taken from Ref. [7].
4.3. Geoneutrinos
Natural radioactivity within the Earth represents a powerful source of heat, influencing the thermal history
of our planet. Radiogenic and primordial Earth heat constitute two of the main contributions to the total energy
loss of the Earth. Available measurements of temperature gradients in mines gives a total thermal production of
46 ± 3 TW [8].
Measuring the ν¯e emitted in the 238U and 232Th radioactive decay chains (geoneutrinos) and testing the
Th/U rate helps in understanding the abundance of radioactive elements and, therefore, the radiogenic heat
contribution and by extension the Earth’s composition (e.g., Ref. [9]).
The potential of the JUNO detector for geoneutrino measurements is due to its depth and radiopurity and,
in particular, to its huge size. The main difficulty comes from the presence of a very significant background,
with respect to the analyzed signal of ν¯e inverse β decay, due to reactor antineutrinos, as shown in Figure 5, where
the expected geoneutrino signal is represented in red.
Visible energy [MeV]
1 2 3 4 5 6 7 8 9 10
Ev
en
t /
 2
25
 ke
V
0
50
100
150
200
250
300
350
400
450
 
Figure 5. Monte Carlo estimate of the prompt IBD candidates for 1 year of measurements, with fixed
chondritic Th/U mass ratio. The expected geo-ν signal is represented in red, and other components
are: the reactor antineutrinos (orange), main background component, 9Li−8 He (green), accidental
(blue). From Ref. [2].
Despite the large reactor antineutrino contribution, one expects an important geoneutrino measurement
at JUNO, which already in the first year of data collection should detect between 300 and 500 events, which is
Universe 2018, 4, 126 7 of 7
more than the number of geoneutrinos detected by that time by all the other available experiments (KamLAND,
Borexino, and SNO+).
5. Conclusions
The JUNO detector, with an unprecedented size, PMT coverage, light yield, and energy resolution, has
a high potential to determine the neutrino mass hierarchy, using neutrino oscillations in vacuum. It is rapidly
progressing through the design, prototyping, and construction phases, and besides its main physics goal, it will
also be able to precisely measure the neutrino oscillation parameters below the 1% level, pursuing a rich neutrino
physics and astrophysics programme. The experiment is scheduled to start data collection in 2021.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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article distributed under the terms and conditions of the Creative Commons Attribution
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