Form 8

Research Result Report
ICRR Inter-University Research Program 2023
Research Subject: Study of supernova neutrinos in Super-Kamiokande
Principal Investigator:

Michel Gonin (ILANCE laboratory, CNRS-University ofTokyo)
Participating Researchers:
Ecole polytechnique universite paris-saclay: Oliver Drapier, Thomas Mueller, PascalPaganini, Margherita Buiza-Avanzini, Benjamin Quilain
Okayama University: Yusuke Koshio, Yota Hino, Masayuki Harada, Seiya Sakai,Fumi Nakanishi, Tomoaki Tada, Tomohiro Tano, Mathieu Lelong, Alexandre Sevestre

Summary of Research Result：
The purpose of this research is detection of the supernova neutrino in SK-Gd. There
are two targets, one is neutrinos from nearby supernova explosion, the other is diffuse
neutrinos from the past supernovae called ‘Supernova Relic Neutrinos (SRN)’. TheSK-Gd project is gadolinium (Gd) loading into Super-Kamiokande (SK) to increase
inverse beta decay interactions of anti-electron neutrinos.
In 2020 summer, we’ve doped 13 tons of gadolinium sulfate into Super-Kamiokande,
which is equivalent to 0.01% Gd mass concentration, and the SK-Gd experiment
officially started. The neutron capture efficiency showed the expected performance,
and the detector was operated stably. In 2022 summer, an additional 26 tons ofgadolinium sulfate was introduced to the detection. (0.03% Gd mass concentration)
The SK-Gd data taking is currently working well.
In FY2023, we reported the first result of supernova relic neutrino in SK-Gd with
0.01% mass concentration gadolinium loaded water in SK-Gd [2]. Though nosignificant signals were observed (left figure), the data showed that similarsensitivity was achieved with 20% live time data compared to the pure water period.
We also showed the atmospheric neutrino – oxygen neutral-current quasielastic(NCQE) cross section [1] (right figure), which is main background events forsupernova relic neutrino search (magenta in left figure). From this analysis, we
obtained new knowledge about the interaction of neutrons with oxygen nuclei.
No.

presents the number
lection criteria exceptdE.

(2 )ction of IBD for eachas a target in the 22.5
is the live time ofaged signal efﬁciency
on criteria are applied
width at each bin. The
Erec + 1.8 MeV. The
ulation in Strumia &he background-only
ted using the number
statistical uncertainty.
vity by replacing N90

7small event fraction, these uncertainties are negligible.
Systematic uncertainty of the measured NCQE crosssection is estimated by performing toy MC considering the systematic uncertainties.
As a result, the 1σ confidence level region becomes [0.59,1.59] × 10−38 cm2 /oxygen, and the measured NCQE
cross section is determined asmeasured⟨σNCQE⟩ = 0.74 ± 0.22(stat.)+0.85−0.15 (syst.)

× 10−38 cm2 /oxygen.

(5)

13

0.9
2.5

0.8
0.7

2

Theoretical
Theoretical (flux-averaged)

1.5

Total

Measured

0.6
0.5

Atmospheric neutrino flux

0.4
1Stat.

0.3

Atmospheric neutrino flux [GeV-1 cm-2 sec-1 sr -1]

applied, 16 eventsin 552.2 day data. In
ns of Erec, of widths
and 23.5–29.5 MeV
nd region is separateds the Erec spectrum of1.
number of events due(p-value) are evaluated
6pseudo experiments
d expected backgroundhe latter. The obtainedde that no signiﬁcantcted background since
e is 25.8%.
of signal excess overnﬁdence level (C.L.;
xperiments using theσ statistical uncertainund events with theire the ﬂux upper limit
Assuming there is noﬂux for each bin isForm 8Cross section [10-38 cm2]or only signal energy
events, spallation 9Li,
mated by Abe et al.
% above 15.49 MeV,
and reactor neutrino
onents, such as nonce events, are newly
VI based on the same%, respectively.

0.2
The measured NCQE cross section, the theoretical
0.5
NCQE cross section [19], and the atmospheric neutrino
0.1
flux predicted using the HKKM11 model [15] are shown
00in Fig. 3. The measured NCQE cross section is con1
100.16
Energy[GeV]sistent with the flux-averaged theoretical NCQE crossFigure2. Reconstructedenergyspectra of the observed
data and thesection
within theuncertainties.
Furthermore,theexpected
meabackgroundafter datacrossreductions
with aislinear(top)energy
andwitha logarithmic(bottom)
Observed
reconstructed
Observed
neutrino
neutrino-oxygen
FIG. 3. The measured
neutrino-oxygen
NCQE cross section,
sured
NCQEsection
consistent
the measurescale for the vertical axis. These include the signal energy region and the sidethe theoretical neutrino-oxygen NCQE cross section [19], andmentin
theSK
pure-waterphase
withintheuncertainties
band region above 29.5 MeV. Each color-ﬁlled histogram shows the expecteddistribution
andpredictedbackground
NCQE
crossneutrino
sectionSK-Gd,
the atmosphericflux inpredictedusing with
the HKKM11+0.78−38 represent
2backgrounds.
The error bars
in(syst.)the data
pointsthe statistical(1.01± 0.17(stat.)
× 10cm/oxygen)
[14].
−0.30
model [15]. Vertical bars show the statistical uncertainty
uncertainty
estimatedbytaking
thesquarerootof
thenumberofobserved
Thesystematic[2].
uncertainty on the measured NCQE crossinTheseSK-Gdtheoreticalprediction[1].(long bar). Horizontal(short bar) and thetotal uncertainty
events.
background histograms are stacked on the other histograms. The
sectionin this thestudy
is larger uncertainty
than thatthehatchedareas represent
total systematicfor ineachbin.measureThe size ofbars show the 1σ from the mean (0.60 GeV) of the theoreticaluncertainty
for theeach SK
backgroundis mentioned
in themain
text. The
dotted–ment ofpure-waterphase.
The
reason
isred
that
we
NCQE cross section multiplied by the atmospheric neutrino
dashed line shows the DSNB expectation from the Horiuchi+09
takethe diﬀerenceofwhichsecondaryinteraction
modelsintoflux.
model(Horiuchietal.
2009),
isdrawn
separately
fromthestacked
Publishedpaper
consideration,conservatively
histogramof the estimatedbackgrounds. estimated by the comparison S.Sakai
among theseetmodels.
The uncertainty will of
be reduced[1]al., Measurement
the neutrino-oxygen neutral-current quasielastic
with better understandingof 1secondary interaction modTable
the SK pure-water phase, and the secondary interaction
Summary
Observed Events, Expected Background Events, and p-value for
els inoffuture.
will be able
to be verifiedPhys.
more precisely.
Adcross
sectionusing
atmospheric neutrinos inmodels
the SK-Gdexperiment,Rev. D 109,
Each Erec Bin
ditional measurement using T2K’s accelerator neutrino(MeV)
Observed
Expectedp-value
ErecL011101(2023).
beam interactions in SK-Gd will help to further refinethe physics models for the secondary interactions.
7.5–9.5
5
7.73 ± 2.54 PROSPECTS
0.798
VII. CONCLUSION
AND FUTURE[2] M.Harada et al., Search for astrophysical electron antineutrinos in Super-

9.5–11.55
4.14 ± 1.23
0.398
11.5–15.5
3
2.13 ± 0.59
0.359
Wereported
thefirstmeasurementof
theatmospheric
15.5–23.5
20.98 ± 0.35
0.258
23.5–29.5
10.98 ± 0.41
0.597
neutrino-oxygen NCQE
cross sectionin the Gd-loaded

Kamiokande with 0.01wt% gadolinium-loaded water,
The Astrophysical JournalACKNOWLEDGMENTS
SK water 951,
Cherenkov
Letters,
L27 detector.
(2023). Using a 552.2 dayNote.
Errorsthe expected
represent
onlymeasured
the systematicdataset,for theNCQE backgroundcross section
wastouncertainty.
+0.85
−382

We gratefully acknowledge the cooperation of thebe
0.74 ± 0.22(stat.) −0.15 (syst.) × 10
cm /oxygen
Kamioka Mining and Smelting Company. The Superin the3 showsenergytherange
fromof 160
to 10 inGeV,
Kamiokande experiment has been built and operated
Figureupper limit
the n̄eMeV
ﬂux extracted
thisPresentations
whichwaswiththe DSNB
atmosphericfrom funding by the Japanese Ministry of Education,
searchwith therangeconsistentof expectationsof modernmodels.
neutrino-flux-averaged
theoreticalNCQE
cross secCulture, supernova
Sports, Scienceneutrino
and Technology,
the U.S. De(1)Harada,Firstofasearchfordiffuse
backgroundinThe
mostM.
optimistic
expectationis result
Kaplinghat+00(Kaplinghattion (1.02 × 10−38 cm2 /oxygen) and the measured
partment ofEnergy, 28
and– theNational Science
SK-Gd experiment, TAUP23, Wienna, Austria,
August
Sep.U.S.
1, 2023.
NCQE cross section in the SK pure-water phase
Foundation. Some of us have been supported by5(2)Sakai, +0.78
Measurement−38 of2 the neutrino-oxygen neutral-current quasielastic cross(1.01 S.
± 0.17(stat.)
cm /oxygen).
funds from the National Research Foundation of Ko−0.30 (syst.) × 10
sectionatmospheric
SK-Gd experiment,
TAUP23,
Wienna,Moreover,fromusing
the comparisonof three neutrinos
diﬀerent sec-in therea (NRF-2009-0083526
and NRF
2022R1A5A1030700)
ondaryinteraction
models, 28
we –found
BIC andfunded by the Ministry of Science, Information andAustria,
August,
Sep.that1, 2023.
INCL++
provide Statusa somewhat
betterfit to the ofobserved
Communication(ICT), Nagoya,
the InstituteJapan,
for Ba(3)Y. Hino,andprospect
the SK-Gdproject,Technology
ICRC2023,
data than BERT.
sic Science (IBS-R016-Y2), and the Ministry of EducaJuly26 – August 3, 2023.
tion (2018R1D1A1B07049158, 2021R1I1A1A01042256),
As described in Sec. II, we continue the observation(4)
Nakanishi,
Evaluationof neutron
tagging
efficiency
0.03%of Gd
mass
theJapan Society
for the on
Promotion
Science,
inwith F.
a 0.03%Gd-loaded SK
detector, the phase
known
concentrationin SK-Gd experiment,
Nagoya,Japan, July26– August
cluding KAKENHI(JP22KJ2301),
the National
Natas SK-VII.
Since the neutron-tagging
eﬃciency in SK- ICRC2023,
ural Science Foundation of China under Grants No.
VII is3,higher
than that in SK-VI (35.6%) [6, 26], more
2023.
11620101004,
the Spanish Ministry of Science,
Univerdelayed
can beSupernova
detected, and theobserved num(5)Y. signals
Koshio,neutrino
detection
in Super-Kamiokande
and Hypersities and Innovation (grant PGC2018-099388-B-I00),
ber of events can be accumulated faster in SK-VII thanKamiokande,
New Physics
Opportunities
atSciences
Neutrino
FacilitiesResearchWorkshop:the Naturaland EngineeringCounin SK-VI.
Assuming that3rdthe neutron-tagging
eﬃciency
AstrophysicalNeutrinos,
SLAC,
USA,
June
11-13,
2023.
cil (NSERC) of Canada, the Scinet and Westgrid conin SK-VII is about 60%, the statistics increases by aboutsortia of Compute Canada, the National Science Centre
1.4 times with the same live time as SK-VI. After one(UMO-2018/30/E/ST2/00441) and the Ministry of Edumore year of observation in SK-VII the statistical uncercation and Science (DIR/WK/2017/05), Poland, the Scitainty will reach the NCQE cross section measurement inNo.