Dual-Baseline Search for Active-to-Sterile Neutrino Oscillations in NOvA

Abstract

We report a search for neutrino oscillations to sterile neutrinos under a model with three active and one sterile neutrinos ( 3 + 1 model). This analysis uses the NOvA detectors exposed to the NuMI beam, running in neutrino mode. The data exposure, 13.6 × 10 20 protons on target, doubles that previously analyzed by NOvA, and the analysis is the first to use ν μ charged-current interactions in conjunction with neutral-current interactions. Neutrino samples in the near and far detectors are fitted simultaneously, enabling the search to be carried out over a Δ m 41 2 range extending 2 (3) orders of magnitude above (below) 1 eV 2 . NOvA finds no evidence for active-to-sterile neutrino oscillations under the 3 + 1 model at 90% confidence level. New limits are reported in multiple regions of parameter space, excluding some regions currently allowed by IceCube at 90% confidence level. We additionally set the most stringent limits for anomalous ν τ appearance for Δ m 41 2 ≤ 3 eV 2 .

Published by the American Physical Society 2025

Full text

Dual-Baseline Search for Active-to-Sterile Neutrino Oscillations in NOvA
M. A. Acero,2B. Acharya,32 P. Adamson,13 N. Anfimov,27 A. Antoshkin,27 E. Arrieta-Diaz,28 L. Asquith,40 A. Aurisano,7
A. Back,21,25 N. Balashov,27 P. Baldi,26 B. A. Bambah,18 E. F. Bannister,40 A. Barros,2A. Bat,3,12 K. Bays,31 R. Bernstein,13
T. J. C. Bezerra,40 V. Bhatnagar,34 D. Bhattarai,32 B. Bhuyan,16 J. Bian,26,31 A. C. Booth,36,40 R. Bowles,21 B. Brahma,19
C. Bromberg,29 N. Buchanan,9A. Butkevich,23 S. Calvez,9T. J. Carroll,48 E. Catano-Mur,47 J. P. Cesar,42 A. Chatla,18
R. Chirco,20 B. C. Choudhary,11 A. Christensen,9M. F. Cicala,44 T. E. Coan,39 A. Cooleybeck,48 C. Cortes-Parra,28
D. Coveyou,45 L. Cremonesi,36 G. S. Davies,32 P. F. Derwent,13 P. Ding,13 Z. Djurcic,1K. Dobbs,17 M. Dolce,46 D. Doyle,9
D. Dueñas Tonguino,7E. C. Dukes,45 A. Dye,32 R. Ehrlich,45 E. Ewart,21 P. Filip,24 M. J. Frank,37 H. R. Gallagher,43
F. Gao,35 A. Giri,19 R. A. Gomes,15 M. C. Goodman,1M. Groh,9R. Group,45 A. Habig,30 F. Hakl,22 J. Hartnell,40
R. Hatcher,13 H. Hausner,48 M. He,17 K. Heller,31 V. Hewes,7A. Himmel,13 T. Horoho,45 A. Ivanova,27 B. Jargowsky,26
J. Jarosz,9M. Judah,9,35 I. Kakorin,27 A. Kalitkina,27 D. M. Kaplan,20 B. Kirezli-Ozdemir,12 J. Kleykamp,32 O. Klimov,27
L. W. Koerner,17 L. Kolupaeva,27 R. Kralik,40 A. Kumar,34 V. Kus,10 T. Lackey,13,21 K. Lang,42 J. Lesmeister,17 A. Lister ,48
J. Liu,26 J. A. Lock,40 M. Lokajicek,24 M. MacMahon,44 S. Magill,1W. A. Mann,43 M. T. Manoharan,8M. Manrique Plata,21
M. L. Marshak,31 M. Martinez-Casales,13,25 V. Matveev,23 B. Mehta,34 M. D. Messier,21 H. Meyer,46 T. Miao,13 V. Mikola,44
W. H. Miller,31 S. Mishra,4S. R. Mishra,38 A. Mislivec,31 R. Mohanta,18 A. Moren,30 A. Morozova,27 W. Mu,13 L. Mualem,5
M. Muether,46 D. Myers,42 D. Naples,35 A. Nath,16 S. Nelleri,8J. K. Nelson,47 R. Nichol,44 E. Niner,13 A. Norman,13
A. Norrick,13 H. Oh,7A. Olshevskiy,27 T. Olson,17 M. Ozkaynak,44 A. Pal,33 J. Paley,13 L. Panda,33 R. B. Patterson,5
G. Pawloski,31 R. Petti,38 R. K. Plunkett,13 L. R. Prais,32 M. Rabelhofer,25,21 A. Rafique,1V. Raj,5M. Rajaoalisoa,7
B. Ramson,13 B. Rebel,48 P. Roy, 46 O. Samoylov,27 M. C. Sanchez,14,25 S. Sánchez Falero,25 P. Shanahan,13 P. Sharma,34
A. Sheshukov,27 A. Shmakov,26 Shivam,16 W. Shorrock,40 S. Shukla,4D. K. Singha,18 I. Singh,11 P. Singh,36,11 V. Singh,4
E. Smith,21 J. Smolik,10 P. Snopok,20 N. Solomey,46 A. Sousa,7K. Soustruznik,6M. Strait,13,31 L. Suter,13 A. Sutton,14,25
K. Sutton,5S. Swain,33 C. Sweeney,44 A. Sztuc,44 B. Tapia Oregui,42 N. Talukdar,38 P. Tas,6T. Thakore,7J. Thomas,44
E. Tiras,12,25 M. Titus,8Y. Torun,20 D. Tran,17 J. Tripathi,34 J. Trokan-Tenorio,47 J. Urheim,21 P. Vahle,47 Z. Vallari,5
J. D. Villamil,28 K. J. Vockerodt,36 M. Wallbank,7,13 C. Weber,31 M. Wetstein,25 D. Whittington,41,21
D. A. Wickremasinghe,13 T. Wieber,31 J. Wolcott,43 M. Wrobel,9S. Wu,31 W. Wu,26 W. Wu,35 Y. Xiao,26 B. Yaeggy,7
A. Yahaya,46 A. Yankelevich,26 K. Yonehara,13 S. Zadorozhnyy,23 J. Zalesak,24 and R. Zwaska13
(NOvA Collaboration)
1Argonne National Laboratory, Argonne, Illinois 60439, USA
2Universidad del Atlantico, Carrera 30 No. 8-49, Puerto Colombia, Atlantico, Colombia
3Bandirma Onyedi Eylül University, Faculty of Engineering and Natural Sciences, Engineering Sciences Department,
10200, Bandirma, Balıkesir, Turkey
4Department of Physics, Institute of Science, Banaras Hindu University, Varanasi, 221 005, India
5California Institute of Technology, Pasadena, California 91125, USA
6Charles University, Faculty of Mathematics and Physics, Institute of Particle and Nuclear Physics, Prague, Czech Republic
7Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
8Department of Physics, Cochin University of Science and Technology, Kochi 682 022, India
9Department of Physics, Colorado State University, Fort Collins, Colorado 80523-1875, USA
10Czech Technical University in Prague, Brehova 7, 115 19 Prague 1, Czech Republic
11Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
12Department of Physics, Erciyes University, Kayseri 38030, Turkey
13Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
14Florida State University, Tallahassee, Florida 32306, USA
15Instituto de Física, Universidade Federal de Goiás, Goiânia, Goiás, 74690-900, Brazil
16Department of Physics, IIT Guwahati, Guwahati, 781 039, India
17Department of Physics, University of Houston, Houston, Texas 77204, USA
18School of Physics, University of Hyderabad, Hyderabad, 500 046, India
19Department of Physics, IIT Hyderabad, Hyderabad, 502 205, India
20Illinois Institute of Technology, Chicago, Illinois 60616, USA
21Indiana University, Bloomington, Indiana 47405, USA
22Institute of Computer Science, The Czech Academy of Sciences, 182 07 Prague, Czech Republic
PHYSICAL REVIEW LETTERS 134, 081804 (2025)
Featured in Physics
0031-9007=25=134(8)=081804(8) 081804-1 Published by the American Physical Society

23Institute for Nuclear Research of Russia, Academy of Sciences 7a, 60th October Anniversary prospect, Moscow 117312, Russia
24Institute of Physics, The Czech Academy of Sciences, 182 21 Prague, Czech Republic
25Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
26Department of Physics and Astronomy, University of California at Irvine, Irvine, California 92697, USA
27Joint Institute for Nuclear Research, Dubna, Moscow region 141980, Russia
28Universidad del Magdalena, Carrera 32 No 22-08 Santa Marta, Colombia
29Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
30Department of Physics and Astronomy, University of Minnesota Duluth, Duluth, Minnesota 55812, USA
31School of Physics and Astronomy, University of Minnesota Twin Cities, Minneapolis, Minnesota 55455, USA
32University of Mississippi, University, Oxford, Mississippi 38677, USA
33National Institute of Science Education and Research, Khurda, 752050, Odisha, India
34Department of Physics, Panjab University, Chandigarh, 160 014, India
35Department of Physics, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
36Particle Physics Research Centre, Department of Physics and Astronomy, Queen Mary University of London,
London E1 4NS, United Kingdom
37Department of Physics, University of South Alabama, Mobile, Alabama 36688, USA
38Department of Physics and Astronomy, University of South Carolina, Columbia, South Carolina 29208, USA
39Department of Physics, Southern Methodist University, Dallas, Texas 75275, USA
40Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, United Kingdom
41Department of Physics, Syracuse University, Syracuse, New York 13210, USA
42Department of Physics, University of Texas at Austin, Austin, Texas 78712, USA
43Department of Physics and Astronomy, Tufts University, Medford, Massachusetts 02155, USA
44Physics and Astronomy Department, University College London, Gower Street, London WC1E 6BT, United Kingdom
45Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA
46Department of Mathematics, Statistics, and Physics, Wichita State University, Wichita, Kansas 67260, USA
47Department of Physics, William & Mary, Williamsburg, Virginia 23187, USA
48Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
(Received 11 September 2024; accepted 6 January 2025; published 26 February 2025)
We report a search for neutrino oscillations to sterile neutrinos under a model with three active and one
sterile neutrinos (3þ1model). This analysis uses the NOvA detectors exposed to the NuMI beam, running
in neutrino mode. The data exposure, 13.6×1020 protons on target, doubles that previously analyzed by
NOvA, and the analysis is the first to use νμcharged-current interactions in conjunction with neutral-
current interactions. Neutrino samples in the near and far detectors are fitted simultaneously, enabling the
search to be carried out over a Δm2
41 range extending 2 (3) orders of magnitude above (below) 1eV2.
NOvA finds no evidence for active-to-sterile neutrino oscillations under the 3þ1model at 90% confidence
level. New limits are reported in multiple regions of parameter space, excluding some regions currently
allowed by IceCube at 90% confidence level. We additionally set the most stringent limits for anomalous ντ
appearance for Δm2
41 ≤3eV2.
DOI: 10.1103/PhysRevLett.134.081804
Neutrino mixing is a well-established phenomenon, with
numerous experiments reporting results that agree with a
picture including three neutrino mass states (ν1,ν2,ν3) that
mix to form three neutrino flavors (νμ,νe,ντ)[1–11]. In this
three-flavor (3F) framework, oscillations are governed
by two mass-squared splittings, Δm2
21 and Δm2
32, where
Δm2
ji ≡m2
j−m2
i, which correspond to the frequency of
oscillation for a given neutrino energy (Eν) and path length
(L), three mixing angles, θ12,θ13 , and θ23, which drive
the magnitude of oscillation, and a CP violating phase,
δCP, which allows for differences between neutrino and
antineutrino oscillations. Over the past two decades, a
number of anomalous results have been reported in short
baseline accelerator neutrino experiments [12,13], radio-
chemical experiments [14–16], and in the reactor neu-
trino sector [17]. If these anomalies are interpreted as
neutrino oscillations, they would require Δm2∼1eV2≫
Δm2
32;Δm2
21, necessitating additional neutrino mass states
be added to the model. Measurements of the width of the Z
boson at the LEP experiments indicate that any additional
neutrinos with mν<m
Z0=2must be sterile [18], meaning
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Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
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PHYSICAL REVIEW LETTERS 134, 081804 (2025)
081804-2

that they do not interact via the weak force. The global
picture of a mass splitting in the ∼1eV2region is compli-
cated by the presence of a number of null results [19–33].
NOvA can probe for active-to-sterile neutrino oscilla-
tions by searching for disappearance of neutral-current
(NC) interactions, which provides a flavor-agnostic meas-
urement of the active neutrino event rate. We can addi-
tionally search for active-to-sterile oscillations among νμ
charged-current (CC) interactions by testing for additional
sources of disappearance when compared to 3F oscilla-
tions. The NOvA experiment consists of two functionally
identical detectors placed 14.6 mrad off-axis of Fermilab’s
NuMI beam [34]. The NuMI beam is extracted over 10 μs
approximately every 1.3 s when 120 GeV protons strike a
graphite target resulting in a secondary hadron beam. These
hadrons are focused using two magnetic horns, and decay
to neutrinos as they travel through a 675 m helium-filled
decay pipe. The off-axis placement of the detectors results
in a narrow neutrino-energy distribution peaked around
2 GeV, with a width of 0.4 GeV and a subdominant high-
energy tail.
The NOvA Near Detector (ND) is positioned at Fermilab
in Batavia, Illinois, 1 km downstream of the NuMI target
and 100 m underground, while the Far Detector (FD) is
located 810 km from the target, at Ash River, Minnesota, on
the surface with a 3 m water-equivalent overburden. The
NOvA detectors are tracking calorimeters with the ND
(FD) constructed of 192 (896) planes of highly reflective
PVC cells measuring 3.9cm × 6.6cm with a length of
3.9 m (15.5 m) [35]. The planes are alternately placed with
horizontal and vertical cells to enable three-dimensional
reconstruction, and are filled with a blend of mineral oil
based liquid scintillator doped with 5% pseudocumene [36].
The ND has additional planes of instrumented cells
separated by steel plates at the rear of the detector (“muon
catcher”) to range out muons. Light produced by charged
particles traversing a cell is collected by a single loop of
wavelength-shifting fiber that spans the length of the cell
and is read out on both ends of the fiber at one end of the
cell by one pixel of a 32 pixel avalanche photodiode.
Custom readout electronics are used to shape and digitize
the signal, and any signal meeting a minimum pulse height
requirement within a 550 μs window around the beam
pulse is saved for offline analysis. The cosmic background
at the FD is sampled by a 10 Hz minimum bias trigger [37].
The simplest extension to 3F mixing is the 3þ1model
[38–42], which introduces seven new parameters: Δm2
41,
θ14,θ24 ,θ34,δ14 ,δ24, and δ34. Under this model, the active
neutrino survival probability can be approximated as [27]
1−Pðνμ→νsÞ≈1−cos4θ14cos2θ34 sin22θ24sin2Δ41
−sin2θ34sin22θ23 sin2Δ31
þ1
2sin δ24 sin θ24 sin 2θ23 sin Δ31;ð1Þ
while the νμsurvival probability can be approximated as
Pðνμ→νμÞ≈1−sin22θ24sin2Δ41
þ2sin22θ23sin2θ24 sin2Δ31
−sin22θ23sin2Δ31 ;ð2Þ
where Δji ≡ðΔm2
jiL=4EνÞ. Exact oscillation probability
calculations are used for the analysis.
Equations (1) and (2) show that sterile neutrinos modify
the magnitude of oscillations at the atmospheric frequency,
Δm2
31, and introduce a new sterile frequency driven by
Δm2
41. At the NOvA FD, the frequency of sterile oscil-
lations at Δm2
41 >0.05 eV2is too high for NOvA to
resolve. These oscillations manifest as a downward nor-
malization shift in the neutrino energy spectrum. At
the shorter baseline of the NOvA ND, energy-dependent
sterile oscillations arise when Δm2
41 >0.5eV2(Fig. 1).
FIG. 1. Oscillation probabilities for (a) active neutrino survival
probability and (b) muon neutrino survival probability vs Eνand
L=Eνwith various model parameters. 3F survival probabilities
are shown in black. Probabilities under the 3þ1model are
shown with Δm2
41 of 0.05 eV2(blue), 0.5eV2(red), 5eV2
(green). As the value of Δm2
41 increases, oscillations happen
over shorter baselines, resulting in noticeable oscillations in the
ND. The shaded regions approximately correspond to the fraction
of neutrinos for each Eνand L=Eνin each detector.
PHYSICAL REVIEW LETTERS 134, 081804 (2025)
081804-3

Fitting both detectors simultaneously extends the region of
parameter space to which we are sensitive compared to
previous analyses [43,44]. In the νμCC and NC channels
considered, NOvA is sensitive to the atmospheric 3F
oscillation parameters, θ23 and Δm2
32, as well as to the
sterile-related parameters θ24,θ34 ,Δm2
41, and δ24 , but not to
θ14 as this analysis does not consider νeappearance.
This analysis uses data collected between February
2014 and March 2020, corresponding to 11.0×1020
(13.6×1020) protons on target (POT) for the ND (FD).
Approximately 0.1×1020 POT of ND NC selected events
were removed from the sample for preanalysis validation,
meaning this sample corresponds to 10.9×1020 POT.
The neutrino flux at the NOvA detectors is determined
using a simulation of particle production and transport
through the beamline using
GEANT
4 9.2p03 [45–47]. The
simulated flux is then corrected using the Package to
Predict the Flux, which modifies the prediction using
external hadron-production data [48]. Neutrino interactions
are simulated within the NOvA detectors using
GENIE
3.0.6
[49,50]. Additional information about the
GENIE
configu-
ration used can be found in [51]. The outgoing particles are
propagated through the detector geometry using
GEANT
4
10.4p02. Custom routines are used to simulate the capture
and transport of scintillation light, as well as the response of
the avalanche photodiodes.
The NC interaction candidates are characterized by
hadronic activity resulting from the transfer of energy
and momentum from the neutrino to the nucleus. The final
state lepton is a neutrino and is not detected. All NC
candidates are required to have a reconstructed vertex, at
least one reconstructed particle, and must cross at least
three contiguous planes.
In the ND, the reconstructed vertex is required to be
contained within a volume with boundaries 80 cm from the
top, bottom, and sides of the detector, 150 cm from the front
face, and 260 cm from the rear face excluding the muon
catcher. All reconstructed particles are required to be
contained within a volume excluding 20 cm from the
top, bottom, and sides of the detector, 150 cm from the front
face, and 50 cm from the rear face excluding the muon
catcher. The more stringent requirements on distance from
the front and rear faces of the detector compared with the
requirements on the other faces are selected to reject
background candidates due to interactions in rock upstream
of the detector and CC beam interactions, respectively.
For FD NC candidates, reconstructed particles are
required to be fully contained within a fiducial volume
with boundaries 100 cm from the top, bottom, and sides of
the detector, and 160 cm from the upstream and down-
stream faces. The cosmic background interaction rate is
significantly higher at the FD than the ND due to a
shallower overburden. Accordingly, rather than placing
explicit requirements on the vertex position, we use this
information along with information about the reconstructed
shower shapes and energy, number of hits, and the trans-
verse momentum fraction to construct a boosted decision
tree focused on rejecting cosmic backgrounds.
Signal events are selected using a convolutional neural
network [52,53], which provides probability scores for
different neutrino flavors based on energy depositions in
the detector. Optimal requirements on the score are deter-
mined by using a figure of merit that considers the syste-
matic and statistical uncertainties on the samples. In the FD,
the requirements on convolutional neural network score and
cosmic rejection score are jointly tuned. Any event passing
the 3F νμCC or νeCC selection [51] is additionally removed
from the sample of NC interaction candidates.
The deposited energy of NC interaction candidates is
estimated by taking a weighted sum of the hadronic and
electromagnetic components of the calorimetric energy in
the detector. An additional bias correction is applied as a
function of total calorimetric energy. The overall NC
energy resolution is 30% [54]. The event selection criteria
and neutrino energy estimator used for the νμCC samples
in the ND and FD are described in [51].
We consider systematic uncertainties on the beam flux,
neutrino interactions, and detector modeling [51]. For this
analysis, we identified two sources of uncertainty that
required custom handling.
Typically for oscillation analyses using NOvA’s extrapo-
lation technique [43,44,51], the ND is assumed to have no
oscillations, so any differences between simulation and data
can be attributed to mismodeling in the simulation. We then
tune cross-section models in the ND simulation to the ND
data, producing a new central value (CV) and suite of
uncertainties. Because sterile neutrinos may induce oscil-
lations in the ND, differences between data and simulation
cannot be attributed to cross-section mismodeling, and so
we use untuned simulation and uncertainties. Because the
meson exchange current component of the simulation is the
least well understood, we have developed shape and
normalization uncertainties for this component based on
the model spread of the Val`encia [55], SuSA [56], and
GENIE
empirical [57] meson exchange current models.
Many NC neutrino candidates selected for this analysis
are produced by kaon decays. Because of the lack of
available hadron-production data, prior analyses assigned
the beam kaon component a 30% normalization uncertainty
in addition to Package to Predict the Flux uncertainties.
We instead constrain this uncertainty with samples not
used in the analyses: a horn-off data sample, which allows
us to probe hadron-production uncertainties without the
complications of the focusing horns, and a sample of
uncontained high-energy muon neutrinos, which gives us
access to the focused kaon peak. We fit for the kaon
component normalization marginalizing over potential
sterile oscillations across the region of parameter space
used in this analysis. This technique results in a 10%
uncertainty on this component.
PHYSICAL REVIEW LETTERS 134, 081804 (2025)
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For each systematic uncertainty we randomly vary model
parameters within their uncertainties to produce a new
systematically fluctuated “universe,”u. A covariance
matrix is constructed,
Ci;j ¼1
UΣU
u¼1NCV
i−Nu
i×NCV
j−Nu
j;ð3Þ
where NiðjÞrepresents the number of events in the ith (jth)
energy bin and Uis the total number of universes. The Ci;j
for each systematic uncertainty are summed, producing a
final systematic covariance matrix.
We use two independent analysis techniques for this
search. Analysis 1 employs a hybrid test statistic combining
a Poisson log-likelihood treatment of statistical uncertain-
ties with a Gaussian multivariate treatment of systematic
uncertainties. A covariance matrix encoding only the syste-
matic uncertainties is used to fit for optimal systematic
weights, sαi, for each oscillation channel αand analysis
bin i. This test statistic is expressed as
χ2¼2X
iSi−OiþOilogOi
Si
þX
ij X
αβ
ðsαi−1ÞC−1
αiβjðsβj−1Þ;ð4Þ
where Oiare the observed data, Si¼Pαsαipαirepresents
the systematic weights applied to the prediction, pαi, and
Cαiβjis a covariance matrix encoding the systematic
uncertainty in each oscillation channel (α,β) and analysis
bin (i,j) and their correlations.
Analysis 2 employs a traditional Gaussian multivariate
formalism,
χ2¼X
iX
j
ðPi−OiÞðCij þVijÞ−1ðPj−OjÞ;ð5Þ
where PiðjÞ(OiðjÞ) is the number of predicted (observed)
events in bin iðjÞ. This analysis adds statistical uncertainties
to the diagonal of the covariance matrix using the com-
bined Neyman-Pearson formalism, Vij ¼f3=½ð1=OiÞþ
ð2=PiÞgδij, which yields a smaller bias in best fit
model parameters than either the Neyman or Pearson
construction [58].
For both analyses the 3F atmospheric oscillation param-
eters θ23 and Δm2
32 are varied in the fit, with a loose
Gaussian constraint applied to Δm2
32 to pin the fit to a 3þ1
flavor paradigm. This constraint is centered at j2.51j
0.15 ×10−3eV2in both mass orderings, and is derived
from a 2020 global fit to data [59] including atmospheric
neutrino oscillations. The width of the constraint is con-
servatively set to double the 3σrange. The sterile param-
eters Δm2
41,θ24 ,θ34, and δ24 are also freely varied when
fitting. The other sterile parameters are held fixed at 0 in the
fit, due to constraints from solar and reactor experiments
[60] and unitarity [61].
We select 2 826 066 (103 109) νμCC (NC) candidates
from the ND data compared with the 3F prediction of
2 450 000 530 000 (115 000 30 000). Additionally,
we select 209 [62] (469) νμCC (NC) candidates from
the FD data, compared with a 3F prediction of 200 45
(471 116) using prefit parameter values Δm2
32 ¼2.51 ×
10−3eV2and θ23 ¼49.6°[59].
The best fits from the two analyses agree well with the
data and with each other (Fig. 2). The technique used by
Analysis 1 allows us to present the best fit with systematic
uncertainty pulls applied (dashed), resulting in improved
agreement between data and simulation. This agreement
indicates that any discrepancy between data and simulation
can be accounted for by our systematic uncertainties.
We present 90% limits for both the Δm2
41 −sin2θ24 and
Δm2
41 −sin2θ34 spaces [Figs. 3(a) and 3(b)]. For each, a
Δχ2surface is constructed over a grid of the two parameters
that define the space, with a fit performed for the remaining
parameters at each point. To correct the critical χ2values
we use a hybrid Feldman-Cousins technique. Covariance
matrix fitting techniques do not result in fitted pull terms for
012345
(GeV)Reconstructed E
0
50
100
150
200
250
Events / 0.1 GeV
3
10
-beam
POT
20
10, 11.0ND CC
110
(GeV)Reconstructed E
0
50
100
150
200
250
Events/GeV
3
10
-beam
POT
20
10ND NC, 10.9
012345
(GeV)Reconstructed E
0
5
10
15
20
25
Events / 0.1 GeV
-beam
POT
20
10, 13.6FD CC
110
(GeV)Reconstructed E
0
100
200
300
Events/GeV
-beam
POT
20
10FD NC, 13.6
Data
Cosmic Background
3F Background
w/ Syst. Uncertainty
3F Total
Analysis 1 Best Fit
(w/ Syst. Pulls)
Analysis 1 Best Fit
Analysis 2 Best Fit
FIG. 2. Reconstructed neutrino energy spectra for selected νμ
CC (left) and NC (right) candidates in the ND (top) and FD
(bottom). NOvA data shown as black points with 3F expectation
shown as a gray histogram (mostly obscured) with shaded
uncertainty bands. The backgrounds are shown as a stacked
histogram in each panel, and are separated into cosmogenic data
and simulated backgrounds under a 3F model with Δm2
32 ¼
2.51 ×10−3eV2and θ23 ¼49.6°. The best fits for Analysis 1 and
Analysis 2 are shown in orange and blue, respectively. The
dashed histogram shows the best fit for Analysis 1 with
systematic pulls applied.
PHYSICAL REVIEW LETTERS 134, 081804 (2025)
081804-5

each systematic uncertainty. Accordingly, for systematic
uncertainties we use a Highland-Cousins technique [63],
where for each new universe a value of each systematic
parameter is drawn from its a priori distribution. For
oscillation parameters, we use the Profiled Feldman-
Cousins technique [64].
NOvA’sΔm2
41 −sin2θ24 limits [Fig. 3(a)] are competi-
tive at high Δm2
41, and exclude new regions of interest
in the IceCube 90% allowed region [65], around 6eV2<
Δm2
41 <11 eV2. Analysis 1 excludes slightly more values
of sin2θ24 at low Δm2
41 and Analysis 2 excludes slightly
more at high Δm2
41. Sensitivity to sin2θ24 primarily comes
from muon neutrino disappearance, which is independent of
θ34 [Eq. (2)]. For high values of Δm2
41 sensitivity is driven by
the ND data, meaning differences in the limits come from
different handling of the systematic uncertainties. Sensitivity
at low Δm2
41 arises primarily from FD data, meaning that the
weaker limit in Analysis 2 comes from undercoverage of the
combined Neyman-Pearson statistical technique.
Our Δm2
41 −sin2θ34 contours [Fig. 3(b)] represent
world-leading limits for Δm2
41 <0.1eV2. Sensitivity to
sin2θ34 comes from our NC samples. For oscillations at the
sterile frequency, oscillation probability ∝cos2θ34 sin2θ24,
resulting in reduced sensitivity to sterile oscillations in the
ND [Eq. (1)]. For this space our sensitivity comes primarily
from oscillations at the atmospheric frequency and there-
fore FD data, which are statistically limited and without
strong dependence on Δm2
41. In this space, Analysis 1
excludes slightly more parameter space than Analysis 2
across the full range considered. The differences in the
contours in this space are attributed to the different
statistical treatments of the two analyses.
Finally, in Fig. 3(c), we present our results in terms of the
effective mixing parameter, sin22θμτ ¼4jUμ4j2jUτ4j2¼
sin2θ24sin2θ34 , which can be thought of as describing
anomalous sterile-driven ντappearance. Because the analy-
ses are consistent and the Feldman-Cousins procedure is
resource intensive, we choose to present this contour using
only Analysis 1. NOvA’s ND is at a higher L=Eνthan other
experiments with limits in this space, meaning that we are
able to probe to lower values of Δm2
41 resulting in the
NOvA 90% limit being world-leading across large areas
below Δm2
41 ¼3eV2. Notably, this limit excludes a new
region of phase space around Δm2
41 ¼1eV2, the preferred
region of Δm2
41 for current anomalies.
In conclusion, an improved search for sterile neutrino
oscillations under the 3þ1oscillation paradigm has
been performed using NOvA data. We use two co-
variance matrix-based techniques that allow us to probe
a wider range of Δm2
41 values than previous NOvA analyses
[43,44]. Differences between the limits for the two analyses
can be taken as an uncertainty due to analysis choices such
as statistical treatment, systematic treatment, binning of the
Δχ2surface, and fitting technique. We find that the NOvA
data are consistent with 3F oscillations at 90% confidence,
and our limits agree with sensitivity studies performed
10 10 10 10 1
24
2
sin
10
10
10
1
10
10
)
2
(eV
2
41
m
-beam
90% CL excluded
NOvA Analysis 1
NOvA Analysis 2
MINOS/MINOS+
CDHS
CCFR
T2K (NH)
T2K (IH)
SciBooNE & MiniBooNE
Super-Kamiokande
90% CL allowed
IceCube
90% CL allowed
IceCube
10 10 1
34
2
sin
10
10
10
1
10
10
)
2
(eV
2
41
m
-beam
90% CL excluded
NOvA Analysis 1
NOvA Analysis 2
Super-Kamiokande
IceCube-DeepCore
MINOS
T2K
10 10 10 10 1
2
2
sin
10
10
10
1
10
10
)
2
(eV
2
41
m
-beam
90% CL excluded
NOvA Analysis 1
CDHS
CCFR
E531
CHORUS
NOMAD
OPERA (NH)
OPERA (IH)
FIG. 3. NOvA’s Feldman-Cousins corrected 90% confidence limits in (a) Δm2
41 −sin2θ24 space, (b) Δm2
41 −sin2θ34 space, and
(c) Δm2
41 −sin22θμτ space with allowed regions and exclusion contours from other experiments [21–33,65]. Regions to the right of open
contours are excluded. Closed contours for SciBooNE/MiniBooNE, CCFR, and CDHS in (a) also denote exclusion regions. For Super-
Kamiokande, a single value of each mixing angle is reported for Δm2
41 ≥0.1eV2[26]. Arrows in (b) represent a constraint on sin2θ34 at
a single value of Δm2
41 [24,27,33]. OPERA NH=IH contours in (c) overlap at Δm2
41 >10−2eV2.
PHYSICAL REVIEW LETTERS 134, 081804 (2025)
081804-6

using 3F oscillations. Our limits are the first presented
in some regions of phase space, while excluding new
regions of parameter space currently allowed by IceCube at
90% confidence level. This Letter additionally sets the
most stringent limits for anomalous ντappearance for
Δm2
41 ≲3eV2, including the strongest limits around
Δm2
41 ¼1eV2.
Acknowledgments—This document was prepared by the
NOvA Collaboration using the resources of the Fermi
National Accelerator Laboratory (Fermilab), a U.S.
Department of Energy, Office of Science, Office of High
Energy Physics HEP User Facility. Fermilab is managed by
Fermi Forward Discovery Group, LLC, acting under
Contract No. 89243024CSC000002. This work was sup-
ported by the U.S. Department of Energy; the U.S. National
Science Foundation; the Department of Science and
Technology, India; the European Research Council; the
MSMT CR, GA UK, Czech Republic; the RAS, Ministry of
Science and Higher Education, and RFBR, Russia; CNPq
and FAPEG, Brazil; UKRI, STFC, and the Royal Society,
United Kingdom; and the state and University of
Minnesota. We are grateful for the contributions of the
staffs of the University of Minnesota at the Ash River
Laboratory, and of Fermilab.
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