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Search for Sub-eV Sterile Neutrinos at RENO

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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 (ν¯e) 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 ν¯e 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≲|Δm412|≲0.5 eV2 region, free from reactor ν¯e flux and spectrum uncertainties. The RENO result provides the most stringent limits on sterile neutrino mixing at |Δm412|≲0.002 eV2 using the ν¯e disappearance channel.
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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
(RENO Collaboration)
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 ¯
νe
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 104jΔm2
41j0.5eV2region,
free from reactor ¯
νeflux and spectrum uncertainties. The RENO result provides the most stringent limits on
sterile neutrino mixing at jΔm2
41j0.002 eV2using the ¯
νedisappearance channel.
DOI: 10.1103/PhysRevLett.125.191801
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 [1]. 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
[27].
An interesting motivation for investigating a sub-eV
sterile neutrino comes from cosmological data. Recent
Planck data [8] 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 [9].
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 [10].
Based on this reports 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
RENOs 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
Published by the American Physical Society under the terms of
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Further distribution of this work must maintain attribution to
the author(s) and the published articles title, journal citation,
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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
reactor ¯
νeinteractions in the RENO experiment. According
to this hypothesis, the survival probability for ¯
νewith an
energy Eand a distance Lis approximately given by [11]
P¯
νe¯
νe1sin22θ13sin2Δ13 sin22θ14 sin2Δ41;ð1Þ
where Δij 1.267Δm2
ijL=E,Δm2
ij m2
im2
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 ¯
νedisappearance
between near and far detectors. In this Letter, RENO
presents a result of the light sterile neutrino search in its
sensitive region of jΔm2
41j0.5eV2.
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 ¯
νeis
detected through the inverse beta decay (IBD) reaction
¯
νeþpeþþn. 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. [1216].
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
104jΔm2
41j0.5eV2. 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% [17]. 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% [17]. 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
jΔm2j1eV2, have a negligible effect on the sterile
neutrino search in RENOs sensitive region of
jΔm2j0.5eV2. 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
response [17].
RENOs multiple reactors provide various baselines
between the near and far detectors for exploring a sterile
neutrino oscillation in a wide range of jΔm2
41jvalues. With
the various baselines and energies of reactor neutrinos, a
sensitivity study for an excluded parameter region is
performed using an Asimov Monte Carlo method [18].
TABLE I. Baselines of near and far detectors from the six
reactor cores.
Baselines (m)
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)
191801-2
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
104<jΔm2
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
jΔm2
41j<104eV2region, an oscillation length becomes
longer than the baseline distance between the two detectors
and loses a search sensitivity. The sensitivity in the 0.01
jΔm2
41j0.5eV2(jΔm2
41j0.01 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
41j0.5eV2region, 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
reactor ¯
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
[19]. The 3þ1neutrino oscillation predictions are also
shown for sin22θ14 ¼0.1and three jΔm2
41jvalues. The
comparison between data and predictions demonstrates
RENOs sensitivity of jΔm2
41j0.5eV2in exploring a
sterile neutrino oscillation. Because of the discrepancy of
observed flux and spectra from the reactor ¯
νemodel
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 [16]. 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. [16]:
χ2¼X
Nbins
i¼1
ðOF=N
iTF=N
iÞ2
UF=N
i
þX
d¼N;Fbd
σd
bkg2
þX
6
r¼1fr
σr
flux2
þϵ
σeff 2
þe
σscale2
;ð2Þ
where OF=N
iand TF=N
iare the observed and expected far-to-
near ratio of IBD events in the ith Epbin, UF=N
iis the
statistical uncertainty of OF=N
i, and OF=N
iis the ratio of the
spectra after background substraction given in Ref. [16].
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-
tainties (σd
bkg,σr
flux,σeff , and σscale ). The details of pull terms
and systematic uncertainties are described in Ref. [16]. The
χ2is minimized with respect to the pull parameters and the
oscillation parameters.
The oscillation parameters of θ14,θ13 , and jΔm2
41jare set
as free. The rest of variables are constrained with
other measurements: sin22θ12 ¼0.846 0.021,Δm2
21 ¼
ð7.53 0.18Þ×105eV2, and jΔm2
32j¼ð2.4440.034Þ×
103eV2[19]. However, the parameters of θ12 and Δm2
21
3
10 2
10 1
10 1
14
θ2
2
sin
4
10
3
10
2
10
1
10
1
)
2
(eV
41
2
mΔ
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 RENOs 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 1.23.0 MeV (3.08.0 MeV) region.
PHYSICAL REVIEW LETTERS 125, 191801 (2020)
191801-3
are fixed because of their negligible effect on χ2. The
parameter Δm2
31 only is constrained by a pull term in the χ2.
The normal mass ordering is assumed for both Δm2
31
and Δm2
41.
The minimum χ2value for the 3þ1neutrino hypothesis
is χ2
4ν=NDF ¼46.4=65, where NDF is the number of
degrees of freedom. The value for the three-neutrino model
with unconstrained jΔm2
31jis χ2
3ν=NDF ¼47.8=66. The
distribution of χ2difference between the two hypotheses,
Δχ2¼χ2
3νχ2
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
jΔm2
41jare set on sterile neutrino oscillation by a standard
Δχ2method [19]. For each parameter set of sin22θ14 and
jΔm2
41j,Δχ2¼χ2χ2
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
41j
are excluded at 95% confidence level if Δχ2is greater than
5.99 [19]. 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 CLs0.05.
The Δχ2and Gaussian CLsmethods obtain 95% C.L.
contours of negligible difference within a statistical
fluctuation.
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. RENOs obtained exclusion contour is mostly
contained in the 1σband.
The fluctuating behavior of the obtained exclusion
contour in the region of jΔm2
41j0.002 eV2comes from
the finite size of the data sample. In the jΔm2
41j
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
41j0.003 eV2as found in the
Asimov study is observed due to an oscillation degeneracy
Prompt Energy (MeV)
12345678
Measured / Expected from Near
0.8
0.9
1
1.1
1.2 2
eV
-2
10× = 5
41
2
mΔ2
eV
-3
10× = 5
41
2
mΔ
2
eV
-3
10× = 1
41
2
mΔ = 0.1 assumed
14
θ2
2
sin
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 [19]. 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.
3
10 2
10 1
10 1
14
θ2
2
sin
4
10
3
10
2
10
1
10
1
)
2
(eV
41
2
mΔ
RENO 95% C.L.
)σ1±RENO 95% C.L. expectation (
FIG. 3. RENOs 95% C.L. exclusion contour for the sterile
neutrino oscillation parameters of sin22θ14 and jΔm2
41j. The
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
excluded.
PHYSICAL REVIEW LETTERS 125, 191801 (2020)
191801-4
of θ13 and θ14. In the jΔm2
41j0.5eV2region, 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
41j0.06 eV2. The uncer-
tainty of background (σd
bkg) is a dominant systematic source
in the 0.003 jΔm2
41j0.06 eV2region, and the energy-
scale uncertainty (σscale) is a major limiting factor in the
jΔm2
41j0.008 eV2region. The uncertainties of flux
(σr
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 [10,2225]. The RENO spectral
comparison between the near and far detectors yields
stringent limits on sin22θ14 in the 104jΔm2
41j
0.5eV2region, while short baseline reactor neutrino
experiments are sensitive to the jΔm2
41j0.01 eV2region.
RENOs1km 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 ¯
νedisappearance
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
104jΔm2
41j0.5eV2. 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
reactor ¯
νeflux and spectrum models. Based on a distinct
reactor complex and RENOs unique systematic uncertain-
ties, it provides an independent result for the sub-eV sterile
neutrino oscillation. Combining it with the Daya Bays
result [10], 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
41j<
0.002 eV2using the ¯
νedisappearance channel.
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-
ces through GSDC, and all the technical and administrative
people who greatly helped in making this experiment
possible.
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PHYSICAL REVIEW LETTERS 125, 191801 (2020)
191801-6
... In the lowest energy range, gallium experiments study the production rate of inverse beta decay on 71 Ga from an intense electron neutrino source [3][4][5][6][7][8]. Also at MeVenergies, reactor experiments have performed searches for the presence of electron antineutrino disappearance by comparing observations to theoretical predictions of the rates [9] or by searching for oscillatory patterns in measurements performed at different positions [10][11][12][13][14][15][16][17][18][19][20]. All these low-energy experiments have yielded confirmatory signals that range in significance from ∼2 to more than 5 sigma but at the same time have yielded constraints that contradict these observations, specially when taking into account solar neutrino analysis [21,22]. ...
... In this work, we focus on the low-energy region, where searches using electron antineutrinos from nuclear reactors and radioactive sources are performed [6,[10][11][12]16,17]. We will show how the plane wave approximation breaks for values of the wave packet size currently allowed [61] and how introducing this formalism produces observable effects. ...
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Light sterile neutrinos have been motivated by anomalies observed in short-baseline neutrino experiments. Among them, radioactive-source and reactor experiments have provided evidence and constraints, respectively, for electron neutrino disappearance compatible with an eV-scale neutrino. The results from these observations are seemingly in conflict. This paper brings into focus the assumption that the neutrino wave packet can be approximated as a plane wave, which is adopted in all analyses of such experiments. We demonstrate that the damping of oscillation due to decoherence effects, e.g., a finite wave packet size, solves the tension between these electron-flavor observations and constraints.
... The Daya Bay collaboration has published an experimental lower limit on the effective flux-averaged neutrino wave packet width [23], though it does not exclude the currently predicted value. Fig. 1 shows the red lines indicating the baselines of each of the reactor experiments described in [57] plus Daya Bay [60], RENO [61], and Double Chooz [62] experiments, alongside the approximate energy spread of the reactor antineutrino spectra. (3) Short baseline experiments: Short baseline neutrino oscillation experiments such as MiniBooNE [63], MicroBooNE [64], SBND [65], and ICARUS [66] (shown as orange lines in Fig. 1) detect neutrinos produced in beams of magnetically focused hadrons created by proton collisions on solid targets. ...
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Models of spontaneous wave function collapse have been postulated to address the measurement problem in quantum mechanics. Their primary function is to convert coherent quantum superpositions into incoherent ones, with the result that macroscopic objects cannot be placed into widely separated superpositions for observably prolonged times. Many of these processes will also lead to loss of coherence in neutrino oscillations, producing observable signatures in the flavor profile of neutrinos at long travel distances. The majority of studies of neutrino oscillation coherence to date have focused on variants of the continuous state localization model, whereby an effective decoherence strength parameter is used to model the rate of coherence loss with an assumed energy dependence. Another class of collapse models that have been proposed posit connections to the configuration of gravitational field accompanying the mass distribution associated with each wave function that is in the superposition. A particularly interesting and prescriptive model is Penrose’s description of gravitational collapse which proposes a decoherence time τ determined through E g τ ∼ ℏ , where E g is a calculable function of the Newtonian gravitational potential. Here we explore application of the Penrose collapse model to neutrino oscillations, reinterpreting previous experimental limits on neutrino decoherence in terms of this model. We identify effects associated with both spatial collapse and momentum diffusion, finding that the latter is ruled out in data from the IceCube South Pole Neutrino Observatory so long as the neutrino wave packet width at production is σ ν , x ≤ 2 × 10 − 12 m . Published by the American Physical Society 2024
... This is evident in figure 1-13, which shows the regions in sin 2 2 ee -∆m 2 parameter space preferred by the RAA and gallium anomalies, as well as global constraints from other experiments. These constraints come from short-to-medium-baseline reactor experiments, including NEOS [94], RENO [95], and Daya Bay [96], as well as very-short-baseline reactor experiments, including STEREO [97], DANSS [98], and PROSPECT [99]. Each of these experiments searches for e disappearance in a reactor-flux-agnostic way: the former though comparisons of the reactor e spectra measured by different detectors [100], and the latter through the use of modular or movable detectors capable of comparing e interaction rates across different baselines. ...
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This thesis covers a range of experimental and theoretical efforts to elucidate the origin of the 4.8σ4.8\sigma MiniBooNE low energy excess (LEE). We begin with the follow-up MicroBooNE experiment, which took data along the BNB from 2016 to 2021. This thesis specifically presents MicroBooNE's search for νe\nu_e charged-current quasi-elastic (CCQE) interactions consistent with two-body scattering. The two-body CCQE analysis uses a novel reconstruction process, including a number of deep-learning-based algorithms, to isolate a sample of νe\nu_e CCQE interaction candidates with 75%75\% purity. The analysis rules out an entirely νe\nu_e-based explanation of the MiniBooNE excess at the 2.4σ2.4\sigma confidence level. We next perform a combined fit of MicroBooNE and MiniBooNE data to the popular 3+1 model; even after the MicroBooNE results, allowed regions in Δm2\Delta m^2-sin22θμe\sin^2 2_{\theta_{\mu e}} parameter space exist at the 3σ3\sigma confidence level. This thesis also demonstrates that the MicroBooNE data are consistent with a νe\overline{\nu}_e-based explanation of the MiniBooNE LEE at the <2σ<2\sigma confidence level. Next, we investigate a phenomenological explanation of the MiniBooNE excess combining the 3+1 model with a dipole-coupled heavy neutral lepton (HNL). It is shown that a 500 MeV HNL can accommodate the energy and angular distributions of the LEE at the 2σ2\sigma confidence level while avoiding stringent constraints derived from MINERν\nuA elastic scattering data. Finally, we discuss the Coherent CAPTAIN-Mills experiment--a 10-ton light-based liquid argon detector at Los Alamos National Laboratory. The background rejection achieved from a novel Cherenkov-based reconstruction algorithm will enable world-leading sensitivity to a number of beyond-the-Standard Model physics scenarios, including dipole-coupled HNLs.
... A number of other experiments have produced results impacting the sterile neutrino interpretations of the Ga anomaly: DANSS [84], Prospect [85], Stéréo [86], RENO & NEOS [87,88] and KATRIN [89] all quote limits and provide exclusion regions. As a collective they exclude most, but not all, of the BEST allowed space. ...
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In order to test the end-to-end operations of gallium solar neutrino experiments, intense electron-capture sources were fabricated to measure the responses of the radiochemical SAGE and GALLEX/GNO detectors to known fluxes of low-energy neutrinos. Such tests were viewed at the time as a cross-check, given the many tests of 71Ge recovery and counting that had been routinely performed, with excellent results. However, the four 51Cr and 37Ar source experiments yielded rates below expectations, a result commonly known as the Ga anomaly. As the intensity of the electron-capture sources can be measured to high precision, the neutrino lines they produce are fixed by known atomic and nuclear rates, and the neutrino absorption cross section on 71Ga is tightly constrained by the lifetime of 71Ge, no simple explanation for the anomaly has been found. To check these calibration experiments, a dedicated experiment BEST was performed, utilizing a neutrino source of unprecedented intensity and a detector optimized to increase statistics while providing some information on counting rate as a function of distance from the source. The results BEST obtained are consistent with the earlier solar neutrino calibration experiments, and when combined with those measurements, yield a Ga anomaly with a significance of approximately 4sigma, under conservative assumptions. But BEST found no evidence of distance dependence and thus no explicit indication of new physics. In this review we describe the extensive campaigns carried out by SAGE, GALLEX/GNO, and BEST to demonstrate the reliability and precision of their experimental procedures, including 71Ge recovery, counting, and analysis. We also describe efforts to define uncertainties in the neutrino capture cross section. With the results from BEST, an anomaly remains.
... The measurement of nonstandard interaction (NSI) in the neutrino sector is an attractive approach to probe beyond-SM physics [2,3]. Current experimental efforts on neutrino NSI are conducted with different neutrino sources, such as reactor neutrinos [4][5][6][7][8][9][10][11][12], accelerator neutrinos [13][14][15][16], and radioactive sources [17][18][19][20][21]. In addition to these terrestrial sources, NSI can also be probed with neutrinos from astrophysical sources, such as stars [22], supernovae [23,24], terrestrial atmosphere [25], and others [26]. ...
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We investigate exotic neutrino interactions using the 205.4-kg · day dataset from the CDEX-10 experiment at the China Jinping Underground Laboratory. New constraints on the mass and couplings of new gauge bosons are presented. Two nonstandard neutrino interactions are considered: a U(1) B-L gauge-boson-induced interaction between an active neutrino and electron/nucleus, and a dark-photon-induced interaction between a sterile neutrino and electron/nucleus via kinetic mixing with a photon. This work probes an unexplored parameter space involving sterile neutrino coupling with a dark photon. New laboratory limits are derived on dark photon masses below 1 eV/c 2 at some benchmark values of Δm 2 41 and g 02 sin 2 2θ 14 .
... The measurement of nonstandard interaction (NSI) in the neutrino sector is an attractive approach to probe beyond-SM physics [2,3]. Current experimental efforts on neutrino NSI are conducted with different neutrino sources, such as reactor neutrinos [4][5][6][7][8][9][10][11][12], accelerator neutrinos [13][14][15][16], and radioactive sources [17][18][19][20][21]. In addition to these terrestrial sources, NSI can also be probed with neutrinos from astrophysical sources, such as stars [22], supernovae [23,24], terrestrial atmosphere [25], and others [26]. ...
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We investigate exotic neutrino interactions using the 205.4−kg·day dataset from the CDEX-10 experiment at the China Jinping Underground Laboratory. New constraints on the mass and couplings of new gauge bosons are presented. Two nonstandard neutrino interactions are considered: a U(1)B–L gauge-boson-induced interaction between an active neutrino and electron/nucleus, and a dark-photon-induced interaction between a sterile neutrino and electron/nucleus via kinetic mixing with a photon. This work probes an unexplored parameter space involving sterile neutrino coupling with a dark photon. New laboratory limits are derived on dark photon masses below 1 eV/c2 at some benchmark values of Δm412 and g′2sin22θ14.
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This Letter presents results of a search for the mixing of a sub-eV sterile neutrino with three active neutrinos based on the full data sample of the Daya Bay Reactor Neutrino Experiment, collected during 3158 days of detector operation, which contains 5.55 × 10 6 reactor ν ¯ e candidates identified as inverse beta-decay interactions followed by neutron capture on gadolinium. The analysis benefits from a doubling of the statistics of our previous result and from improvements of several important systematic uncertainties. No significant oscillation due to mixing of a sub-eV sterile neutrino with active neutrinos was found. Exclusion limits are set by both Feldman-Cousins and CLs methods. Light sterile neutrino mixing with sin 2 2 θ 14 ≳ 0.01 can be excluded at 95% confidence level in the region of 0.01 eV 2 ≲ | Δ m 41 2 | ≲ 0.1 eV 2 . This result represents the world-leading constraints in the region of 2 × 10 − 4 eV 2 ≲ | Δ m 41 2 | ≲ 0.2 eV 2 . Published by the American Physical Society 2024
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The RENO experiment reports more precisely measured values of θ13 and |Δmee2| using ∼2200 live days of data. The amplitude and frequency of reactor electron antineutrino (ν¯e) oscillation are measured by comparing the prompt signal spectra obtained from two identical near and far detectors. In the period between August 2011 and February 2018, the far (near) detector observed 103 212 (850 666) ν¯e candidate events with a background fraction of 4.8% (2.0%). A clear energy and baseline dependent disappearance of reactor ν¯e is observed in the deficit of the measured number of ν¯e. Based on the measured far-to-near ratio of prompt spectra, we obtain sin22θ13=0.0896±0.0048(stat)±0.0047(syst) and |Δmee2|=[2.68±0.12(stat)±0.07(syst)]×10−3 eV2.
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The Reactor Experiment for Neutrino Oscillation (RENO) has been taking electron antineutrino (νe\overline{\nu}_{e}) data from the reactors in Yonggwang, Korea, using two identical detectors since August 2011. Using roughly 500 live days of data through January 2013 we observe 290,775 (31,514) reactor νe\overline{\nu}_{e} candidate events with 2.8 (4.9)% background in the near (far) detector. The observed visible νe\overline{\nu}_{e} spectra in both detectors show discrepancy around 5 MeV with regard to the prediction from the current reactor model. Based on a far-to-near ratio measurement using the spectral and rate information we have obtained sin22θ13=0.082±0.009(stat.)±0.006(syst.)\sin^2 2 \theta_{13} = 0.082 \pm 0.009({\rm stat.}) \pm 0.006({\rm syst.}) and Δmee2=[2.620.23+0.21(stat.)0.13+0.12(syst.)]×103|\Delta m_{ee}^2| =[2.62_{-0.23}^{+0.21}({\rm stat.})_{-0.13}^{+0.12}({\rm syst.})]\times 10^{-3}eV2^2.
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We describe a method based on the CLs approach to present results in searches of new physics, under the condition that the relevant parameter space is continuous. Our method relies on a class of test statistics developed for non-nested hypotheses testing problems, denoted by ΔT, which has a Gaussian approximation to its parent distribution when the sample size is large. This leads to a simple procedure of forming exclusion sets for the parameters of interest, which we call the Gaussian CLs method. Our work provides a self-contained mathematical proof for the Gaussian CLs method that explicitly outlines the required conditions. These conditions are milder than that required by Wilks' theorem to set confidence intervals (CIs). We illustrate the Gaussian CLs method in an example of searching for a sterile neutrino, where the CLs approach was rarely used before. We also compare data analysis results produced by the Gaussian CLs method and various CI methods to showcase their differences.