Search for Sub-eV Sterile Neutrinos at RENO
J. H. Choi,1H. I. Jang,2J. S. Jang,3S. H. Jeon,4K. K. Joo,5K. Ju,6D. E. Jung,4J. G. Kim,4J. H. Kim,4J. Y. Kim,5
S. B. Kim,4S. Y. Kim,7W. Kim,8E. Kwon,4D. H. Lee,4H. G. Lee,7I. T. Lim,5D. H. Moon,5M. Y. Pac,1
H. Seo,7J. W. Seo,4C. D. Shin,5B. S. Yang ,9J. Yoo,9,6 S. G. Yoon,6I. S. Yeo,5and I. Yu4
1Institute for High Energy Physics, Dongshin University, Naju 58245, Korea
2Department of Fire Safety, Seoyeong University, Gwangju 61268, Korea
3GIST College, Gwangju Institute of Science and Technology, Gwangju 61005, Korea
4Department of Physics, Sungkyunkwan University, Suwon 16419, Korea
5Institute for Universe and Elementary Particles, Chonnam National University, Gwangju 61186, Korea
6Department of Physics, KAIST, Daejeon 34141, Korea
7Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea
8Department of Physics, Kyungpook National University, Daegu 41566, Korea
9Institute for Basic Science, Daejeon 34047, Korea
(Received 16 June 2020; revised 8 October 2020; accepted 9 October 2020; published 6 November 2020)
We report a search result for a light sterile neutrino oscillation with roughly 2200 live days of data in the
RENO experiment. The search is performed by electron antineutrino ( ¯
νe) disappearance taking place
between six 2.8 GWth reactors and two identical detectors located at 294 m (near) and 1383 m (far) from the
center of the reactor array. A spectral comparison between near and far detectors can explore reactor ¯
oscillations to a light sterile neutrino. An observed spectral difference is found to be consistent with that of
the three-flavor oscillation model. This yields limits on sin22θ14 in the 10−4≲jΔm2
free from reactor ¯
νeflux and spectrum uncertainties. The RENO result provides the most stringent limits on
sterile neutrino mixing at jΔm2
41j≲0.002 eV2using the ¯
There remain unknown properties of neutrinos even with
impressive progress in neutrino physics. The existence of
sterile neutrinos has been a mysterious and unsolved
problem . The discovery of sterile neutrinos would
not only open a new window into particle physics beyond
the standard model, it would also solve the longstanding
problem of dark matter in the Universe. Almost all the
experimental results indicate that the number of light
neutrino species is consistent with only three flavors.
However, some experimental results may not be explained
by the three active flavor neutrino hypothesis and suggest
an additional flavor of neutrino with a mass around 1 eV
An interesting motivation for investigating a sub-eV
sterile neutrino comes from cosmological data. Recent
Planck data  seem to rule out an additional neutrino
species with a mass near 1 eV assuming full thermalization
in the early Universe. However, sterile neutrinos have
played an important role in explaining the dark radiation
excess and the preference for a hot dark matter component
with mass in the sub-eV range .
Recently, a combined data analysis by the MINOS+ and
Daya Bay Collaborations reported a null observation of the
sterile neutrino oscillations in the sub-eV region .
Based on this report’s significant implication, an indepen-
dent search with distinct detectors and neutrino sources is
desirable for a chance of discovery or verification.
Currently, searches for the light sterile neutrino oscillation
are possible only by reactor experiments having baselines
of ∼1km. RENO has performed a sub-eV sterile neutrino
search that relies on the comparison of spectra measured by
two identical detectors at different locations in order to find
a spectral modulation due to the oscillation. By employing
RENO’s energy calibration, background subtraction, event
selection, and detector performance, this analysis provides
an independent search with unique experimental uncer-
tainties and a distinct reactor complex.
This Letter reports a search for a light sterile neutrino
based on the 3þ1neutrino hypothesis using more than 106
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PHYSICAL REVIEW LETTERS 125, 191801 (2020)
0031-9007=20=125(19)=191801(6) 191801-1 Published by the American Physical Society
νeinteractions in the RENO experiment. According
to this hypothesis, the survival probability for ¯
energy Eand a distance Lis approximately given by 
νe≈1−sin22θ13sin2Δ13 −sin22θ14 sin2Δ41;ð1Þ
where Δij ≡1.267Δm2
jis the mass-
squared difference between the mass eigenstates. This
indicates that the sterile neutrino oscillation with a mixing
angle θ14 introduces an additional spectral distortion by a
squared mass difference jΔm2
41j. Thus, these oscillation
parameters can be explored by a model independent
spectral comparison of the reactor ¯
between near and far detectors. In this Letter, RENO
presents a result of the light sterile neutrino search in its
sensitive region of jΔm2
The RENO experiment uses two identical near and far
detectors located at 294 and 1383 m, respectively, from the
center of six reactor cores at the Hanbit Nuclear Power
Plant Complex in Yonggwang. The near (far) underground
detector has 120 m (450 m) of water equivalent overburden.
Six pressurized water reactors, each with maximum thermal
output of 2.8 GWth, are situated in a linear array spanning
1.3 km with equal spacings. The reactor flux-weighted
baseline is 410.6 m for the near detector and 1445.7 m for
the far detector, respectively. The baseline distances
between the detectors and reactors are measured to an
accuracy or better than 10 cm using the Global Positioning
System and the total station system.
Each RENO detector consists of a main inner detector
filled with 16 tons of 0.1% gadolinium (Gd) loaded liquid
scintillator and an outer veto detector. A reactor ¯
detected through the inverse beta decay (IBD) reaction
νeþp→eþþn. Backgrounds are efficiently removed by
time coincidence between a prompt signal and a delayed
signal from neutron capture on Gd. The prompt signal
releases energy of 1.02 MeVas two γrays from the positron
annihilation in addition to the positron kinetic energy. The
delayed signal produces several γrays with the total energy
of ∼8MeV. The details of the RENO detector are
described in Refs. [12–16].
Because of various baselines between two detectors
and six reactor cores, ranging from 300 m to nearly
1.5 km as shown in Table I, this search is sensitive to
mixing between active and sterile neutrinos in the region of
41j≲0.5eV2. These mixing parameters can
produce an additional modulation in energy with a fre-
quency different from the active neutrino oscillation.
This analysis uses roughly 2200 live days of data taken
in the period between August 2011 and February 2018.
Applying the IBD selection criteria yields 850 666
(103 212) ¯
νecandidate events with the energy of prompt
event (Ep) between 1.2 and 8.0 MeV in the near (far)
detector. The remaining backgrounds are either uncorre-
lated or correlated IBD candidates due to random associ-
ation between the prompt and delayed events, fast neutrons,
and β-n emitters from cosmic-muon induced 9Li=8He
isotopes. The remaining background rates and spectral
shapes are obtained from control data samples [16,17]. The
background fraction for the near (far) detector is 2.0%
(4.8%). The Epenergy scale is determined by various
radioactive sources and neutron capture events. A nonlinear
response of scintillation to the prompt energy due to a
quenching effect and Cherenkov radiation is well under-
stood by the energy calibration. The Epresolution in the
range of 1 to 8 MeV is 8% to 3%. A detailed description of
IBD event selection, their systematic uncertainties, and
background estimation can be found in Refs. [12,16,17].
The uncertainty in the absolute energy scale is estimated
to be 1.0% . This sterile neutrino search based on the
relative measurement of spectra at two identical detectors is
almost insensitive to the uncertainty. On the other hand, the
Epdifference between the near and far detectors makes the
largest contribution to the uncertainty associated with this
analysis. The relative energy-scale difference is estimated
by comparing near and far spectra of calibration data and is
found to be less than 0.15% . This search is rather
insensitive to the rest of systematic uncertainties because of
their relatively minimal energy dependence. The uncorre-
lated reactor-flux uncertainty is 0.9%, the uncorrelated
detection efficiency uncertainty is 0.24%, and the back-
ground uncertainty is 5.61% and 3.26% for the far and near
detectors, respectively. These uncertainties contribute to the
error of a relative rate measurement but minimally to that of
this relative spectral-shape analysis.
The finite sizes of the reactor cores and the antineutrino
detectors, relevant to a search in the region of
jΔm2j∼1eV2, have a negligible effect on the sterile
neutrino search in RENO’s sensitive region of
jΔm2j≲0.5eV2. The expected rates and spectra of reactor
νeare calculated for the duration of physics data taking
by taking into account the varying thermal powers,
fission fractions of four fuel isotopes, energy release per
fission, fission spectra, IBD cross sections, and detector
RENO’s multiple reactors provide various baselines
between the near and far detectors for exploring a sterile
neutrino oscillation in a wide range of jΔm2
the various baselines and energies of reactor neutrinos, a
sensitivity study for an excluded parameter region is
performed using an Asimov Monte Carlo method .
TABLE I. Baselines of near and far detectors from the six
Detectors R1 R2 R3 R4 R5 R6
Near 660 445 302 339 520 746
Far 1564 1461 1398 1380 1409 1483
PHYSICAL REVIEW LETTERS 125, 191801 (2020)
The sample is generated without statistical or systematic
fluctuations assuming the three-neutrino hypothesis.
Figure 1shows an Asimov expected exclusion contour
obtained from a search for a sterile neutrino oscillation by a
far-to-near ratio method, which is described later. In the
41j<0.5eV2region, a relative spectral dis-
tortion between the two detectors occurs and provides
search sensitivity. The dip structure at 0.003 eV2is caused
by a degenerate oscillation effect due to θ13 and θ14. In the
41j<10−4eV2region, an oscillation length becomes
longer than the baseline distance between the two detectors
and loses a search sensitivity. The sensitivity in the 0.01 ≲
41j≲0.01 eV2) region comes from
the spectral comparison at relatively short (long) baselines
between the two detectors or from the prompt energy above
(below) 3 MeV. In the jΔm2
41j≳0.5eV2region, the far-to-
near ratio method is unable to exclude any parameter region
because of no relative spectral distortion between the two
detectors. A rapid oscillation takes place before the near
detector in the large jΔm2
41jregion and generates no
spectral distortion between the two detectors. However,
comparison of their event rates becomes sensitive to
exclude oscillation parameters.
This sterile neutrino search is based on comparison of
observed spectra, with two identical detectors having
different baselines, and thus reduces dependence on a
νeflux and spectrum model. A sterile neutrino
oscillation causes ¯
νedisappearance according to Eq. (1) and
produces relative spectral distortion between the near and
far detectors. Figure 2shows the ratio of the observed
prompt energy spectrum at far detector and the three-
neutrino best-fit prediction from the near detector spectrum
. The 3þ1neutrino oscillation predictions are also
shown for sin22θ14 ¼0.1and three jΔm2
comparison between data and predictions demonstrates
RENO’s sensitivity of jΔm2
41j≲0.5eV2in exploring a
sterile neutrino oscillation. Because of the discrepancy of
observed flux and spectra from the reactor ¯
prediction, this analysis employs the relative spectral
distortion between identical near and far detectors.
Moreover, the spectral ratio comparison cancels out
common systematic uncertainties between the two identical
detectors. The active and sterile oscillation parameters are
determined by a fit to the measured far-to-near ratio of IBD
prompt spectra in the same manner as the previous three-
neutrino oscillation analysis . To find the best fit, a χ2
with pull parameter terms of systematic uncertainties is
constructed using the spectral ratio measurement and is
minimized by varying the oscillation parameters and pull
parameters as described in Ref. :
iare the observed and expected far-to-
near ratio of IBD events in the ith Epbin, UF=N
statistical uncertainty of OF=N
i, and OF=N
iis the ratio of the
spectra after background substraction given in Ref. .
The expected far-to-near ratio is calculated using reactor
and detector information, including pull parameters (bd,fr,
ϵ, and e). The systematic uncertainty sources are embedded
by these pull parameters with associated systematic uncer-
flux,σeff , and σscale ). The details of pull terms
and systematic uncertainties are described in Ref. . The
χ2is minimized with respect to the pull parameters and the
The oscillation parameters of θ14,θ13 , and jΔm2
as free. The rest of variables are constrained with
other measurements: sin22θ12 ¼0.846 0.021,Δm2
ð7.53 0.18Þ×10−5eV2, and jΔm2
10−3eV2. However, the parameters of θ12 and Δm2
All reactors and energy
Reactors 1, 2, 5 and 6
Reactors 3 and 4
1.2 - 3.0 MeV
3.0 - 8.0 MeV
RENO's expected sensitivity
for sterile neutrino search
RENO 95% C.L.
FIG. 1. Expected 95% C.L. exclusion contours from sterile
neutrino searches. The black solid contour represents an expected
limit on ¯
νedisappearance using RENO’s 2200 days of data. The
red solid (dotted) contour represents an exclusion sensitivity
originating from a relatively long (short) baseline search. The
blue solid (dotted) contour represents an exclusion sensitivity
coming from a search in the 1.2–3.0 MeV (3.0–8.0 MeV) region.
PHYSICAL REVIEW LETTERS 125, 191801 (2020)
are fixed because of their negligible effect on χ2. The
31 only is constrained by a pull term in the χ2.
The normal mass ordering is assumed for both Δm2
The minimum χ2value for the 3þ1neutrino hypothesis
4ν=NDF ¼46.4=65, where NDF is the number of
degrees of freedom. The value for the three-neutrino model
with unconstrained jΔm2
3ν=NDF ¼47.8=66. The
distribution of χ2difference between the two hypotheses,
4ν, is obtained from a number of simulated
experiments with a statistical variation and their χ2fits with
systematic uncertainties taken into account. The pvalue
corresponding to the Δχ2value is obtained to be 0.87 for
Δχ2¼1.4. This indicates the data are found to be con-
sistent with the three-neutrino model and show no signi-
ficant evidence for a sterile neutrino oscillation.
Exclusion limits in a parameter space of sin22θ14 and
41jare set on sterile neutrino oscillation by a standard
Δχ2method . For each parameter set of sin22θ14 and
min is obtained, where χ2
min is the
minimum χ2out of all possible parameter sets. The χ2of
each parameter set is obtained by minimizing with varying
θ13 and jΔm2
31j. The parameter sets of sin22θ14 and jΔm2
are excluded at 95% confidence level if Δχ2is greater than
5.99 . Figure 3shows an exclusion contour obtained
from the RENO data. We repeat obtaining exclusion
contours using the Gaussian CLsmethod [20,21]. For each
set of sin22θ14 and jΔm2
41j, this method calculates pvalues
for the three-neutrino and 3þ1neutrino hypotheses and
determines a CLsvalue from them. A 95% C.L. exclusion
region is obtained by requiring a condition of CLs≤0.05.
The Δχ2and Gaussian CLsmethods obtain 95% C.L.
contours of negligible difference within a statistical
In order to understand the validity of the data analysis, a
number of pseudoexperiments are generated within
statistical fluctuation and without the sterile neutrino
hypothesis. Exclusion contours for the pseudoexperiments
are obtained by the same Δχ2method as described above
by taking into account the systematic uncertainties.
Figure 3also shows an expected 1σband of 95% C.L.
exclusion contours due to a statistical fluctuation and its
median. RENO’s obtained exclusion contour is mostly
contained in the 1σband.
The fluctuating behavior of the obtained exclusion
contour in the region of jΔm2
41j≳0.002 eV2comes from
the finite size of the data sample. In the jΔm2
0.002 eV2region, the spectral distortion appears in the
low energy range and gradually disappears. The data
exclude a larger range of sin22θ14 values than the
Asimov prediction in this jΔm2
41jregion. The spectral
deviation from the three-neutrino prediction at low energy
happens to be minimal and obtains a more excluded region
than the most probable expectation. According to pseu-
doexperiments, such an exclusion contour away from the
expectation is estimated to have a probability of roughly
20%. A dip structure at jΔm2
41j∼0.003 eV2as found in the
Asimov study is observed due to an oscillation degeneracy
Prompt Energy (MeV)
Measured / Expected from Near
10× = 5
10× = 5
10× = 1
mΔ = 0.1 assumed
Data predictionνUncertainty of 3
FIG. 2. Prompt energy spectra observed at far detector divided
by the three-neutrino best-fit prediction from the near detector
spectrum . The gray band represents the statistical uncertainty
of the near data and all the systematic uncertainties. Predictions
with sin22θ14 ¼0.1and three jΔm2
41jrepresentative values are
also shown as the blue, red, and green curves.
RENO 95% C.L.
)σ1±RENO 95% C.L. expectation (
FIG. 3. RENO’s 95% C.L. exclusion contour for the sterile
neutrino oscillation parameters of sin22θ14 and jΔm2
black solid contour represents an excluded region obtained from
spectral distortion between near and far detectors. The green
shaded band represents expected 1σexclusion contours due to a
statistical fluctuation. The blue dotted contour represents its
median. The parameter region in the right side of the contours is
PHYSICAL REVIEW LETTERS 125, 191801 (2020)
of θ13 and θ14. In the jΔm2
41j≳0.5eV2region, the spectral
distortion due to the sterile neutrino oscillation is averaged
out before the near detector and a search sensitivity is lost.
The limit of sin22θ14 is mostly determined by a
statistical uncertainty, while the systematic uncertainties
become considerable in the jΔm2
41j≲0.06 eV2. The uncer-
tainty of background (σd
bkg) is a dominant systematic source
in the 0.003 ≲jΔm2
41j≲0.06 eV2region, and the energy-
scale uncertainty (σscale) is a major limiting factor in the
41j≲0.008 eV2region. The uncertainties of flux
flux) and detection efficiency (σeff) have negligible effect
on this analysis.
Figure 4shows exclusion contours obtained from the
RENO data and other experiments as well as an allowed
region from Planck data [10,22–25]. The RENO spectral
comparison between the near and far detectors yields
stringent limits on sin22θ14 in the 10−4≲jΔm2
0.5eV2region, while short baseline reactor neutrino
experiments are sensitive to the jΔm2
RENO’s∼1km baselines allow sensitivity to search for the
sub-eV sterile neutrino mixing. Combining the RENO
result with those of other experiments can improve the
sterile neutrino search sensitivity. More accurate short
baseline reactor and accelerator neutrino experiments are
desirable in order to probe the jΔm2
41jlarger than 0.5eV2.
In summary, RENO reports results from a search for a
sub-eV sterile neutrino oscillation in the ¯
channel using 2200 days of data. No evidence for a sub-eV
sterile neutrino oscillation is found using two identical
detectors and thus yields a 95% C.L. limit on sin22θ14 in
41j≲0.5eV2. RENO obtains a significant
excluded area of sub-eV sterile neutrino oscillation param-
eters by comparison of the measured IBD spectra using two
identical detectors. The search minimizes dependence on
νeflux and spectrum models. Based on a distinct
reactor complex and RENO’s unique systematic uncertain-
ties, it provides an independent result for the sub-eV sterile
neutrino oscillation. Combining it with the Daya Bay’s
result , one can firmly conclude there is no mixing
between sub-eV sterile neutrino and ¯
νein the excluded
parameter region. The RENO result provides the most
stringent limits on sterile neutrino mixing at jΔm2
0.002 eV2using the ¯
The RENO experiment is supported by the National
Research Foundation of Korea (NRF), Grant Nos. 2009-
0083526, 2019R1A2C3004955, and 2017R1A2B4011200
funded by the Korea Ministry of Science and ICT. Some of
us have been supported by a fund from the BK21 of NRF
and Institute for Basic Science (IBS-R017-D1-2020-a00/
IBS-R017-G1-2020-a00). We gratefully acknowledge the
cooperation of the Hanbit Nuclear Power Site and the
Korea Hydro and Nuclear Power Co., Ltd. (KHNP). We
thank KISTI for providing computing and network resour-
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