Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam
muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam
The OPERA collaboration
The OPERA experiment, exposed to the CERN to Gran Sasso νμ beam, collected data from 2008 to 2012. Four oscillated ντ Charged Current interaction candidates have been detected in appearance mode, which are consistent with νμ → ντ oscillations at the atmospheric Δm2 within the “standard” three-neutrino framework. In this paper, the OPERA ντ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino.
Oscillation; Neutrino Detectors and Telescopes; Beyond Standard Model
1 Introduction 2 3 4
Detector, beam, and data sample
The OPERA collaboration
The OPERA experiment  operated in the CERN Neutrinos to Gran Sasso (CNGS)
beam produced at CERN and directed towards the Gran Sasso Underground Laboratory
of INFN (LNGS), 730 km away, where the detector is located. The experiment is unique in
CC interaction candidate events [4–7], consistent with the expectation of the standard
ososcillation in appearance mode.
In the present paper, limits are derived on the existence of a massive sterile neutrino.
so-called reactor  and Gallium [11, 12] neutrino anomalies are also interpreted as due
to the existence of a fourth sterile neutrino with mass at the eV scale. In relation to this
issue, it is worth mentioning that the effective number of neutrino-like species decoupled
from the primeval plasma measured by the Planck collaboration is 3.15 ± 0.23 at 95%
Confidence Level (CL) .
In the following, a short description of the OPERA experimental setup and of the
proregions in the parameter space are derived.
Detector, beam, and data sample
In OPERA, CNGS neutrinos interacted in a massive target made of lead plates interspaced
with nuclear emulsion films acting as high accuracy tracking devices . This kind of
detector is historically called Emulsion Cloud Chamber (ECC). The OPERA detector is
made of two identical Super Modules, each consisting of a target section and a muon
magnetic spectrometer. Each target section has a mass of about 625 tons and is made of
the beam direction corresponds to about 10 radiation lengths. The bricks are assembled
in vertical walls instrumented with scintillator strips (Target Tracker detectors, TT) to
trigger the read-out and locate neutrino interactions within the target.
of the four beam components at the detector site are shown in figure 1 .
The data taking, based on a minimum bias interaction trigger from the TT scintillators,
started in 2008 and ended in December 2012. OPERA collected data corresponding to
17.97 × 1019 protons on target (pot) with 19505 recorded events. The data sample used in
this analysis is defined following the selection criteria described in  and corresponds to
about 75% of the total statistics.
Bricks selected as candidates to contain CNGS neutrino interactions are analysed following
the procedure described in detail in . Here we just recall the main steps of the analysis.
The brick where a neutrino interaction occurred is predicted by the electronic detectors
and extracted from the target by an automatic brick manipulator system. Two extra low
background emulsion films (Changeable Sheets, CS)  located downstream of the brick
act as an interface between the brick and the electronic detectors. If the measurement of
the CS yields tracks related to the neutrino interaction, the emulsion films of the brick are
developed. Their analysis provides the three dimensional reconstruction of the neutrino
interaction and of the possible decay vertices of short-lived particles  with micrometric
expected background in the analysed sample amounts to 0.23 ± 0.05 events. The absence
oscillations in the atmospheric sector, computed within a simplified two-flavour scheme
is 2.30 ± 0.46 (2.21 ± 0.44) assuming normal (inverted) hierarchy of neutrino masses; the
2.43 × 10−3 eV2 (|Δm2| = 2.38 × 10−3 eV2) , where Δm2 is defined as m23 −
number is obtained by rescaling the value given in  for |Δm232| ≈ |Δm231| ≈ |(Δmm21+2m| 22=).
By including the background, 2.53 ± 0.46 (2.44 ± 0.44) events are expected in total. The
atmospheric oscillation parameters. The observation of four events is compatible with
In presence of a fourth sterile neutrino with mass m4, the oscillation probability is a
function of the 4 × 4 mixing matrix U and of the three squared mass differences. Defining
C = 2|Uμ3||Uτ3|, Δij = 1.27 Δmi2j L/E (i,j = 1,2,3,4), φμτ =
P (E) = C2 sin2 Δ31 + sin2 2θμτ sin2 Δ41
− C sin 2θμτ sin φμτ sin2 Δ31 sin 2Δ41
Observed neutrino oscillation anomalies , if interpreted in terms of one additional
values, since negative values are disfavoured by results on the sum of neutrino masses
respectively. The oscillation probability P (E) can thus be approximated to :
factor proportional to the fraction of the analysed sample and to the target mass.
2 distribution of the log
likelihood ratio test statistics: q = −2 ln(Le(φμτ , sin2 2θμτ )/L0 ), where L0 = e−n nn /n!
In figure 2(a) the 90% CL exclusion limits are presented for both normal and inverted
figure 3 together with the unitarity bound (|Uμ4|2 + |Uτ4|2 ≤ 1).
Kamiokande collaboration recently published new limits based on the analysis of
atmo0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
(dashed line) and for the profile likelihood (continuous line).
The unitarity bound (grey line) is also shown. Bands are drawn to indicate the excluded regions.
OPERA NH OPERA IH CHORUS NOMAD
normal (NH, dashed red) and inverted (IH, solid blue) hierarchy of the three standard neutrino
masses. The exclusion plots by NOMAD  and CHORUS  are also shown. Bands are drawn
to indicate the excluded regions.
ments: CDHS , CCFR , MiniBooNE+SciBooNE  and MINOS , as well as by
neutral current interaction rate comparison between the near and the far detector  and
more recently by the Super-Kamiokande analysis on matter effects for up-going atmospheric
ues, the likelihood has been computed using the GLoBES software [30, 31], which takes
approximated by a constant value estimated with the PREM [32, 33] onion shell model.
tions at short-baselines obtained by the NOMAD  and CHORUS  experiments are
dard neutrino masses, with a 10% systematic error deriving from the uncertainties on the
10−3 eV2 would disappear.
A narrow region is excluded at 90% CL at Δm241 ≈ 10−3 eV2 for the normal hierarchy
probability due to the presence of the sterile neutrino. Instead, the oscillation probability
the analysis was repeated following this assumption. The exclusion plots obtained in this
way are similar to those of figure 4, but with hierarchies exchanged. It is worth underlining
that the results obtained in the 3+1 model, shown in figures 2 and 3, are independent of
P (E|CP conservation) ≈ sin2 2θeff · sin2(1.27 Δme2ff L/E)
In this framework, with 4 observed events and 2.53 ± 0.46 events expected from the normal
hierarchy of standard oscillations, including background, the upper limit on the number of
pearance at a baseline of 730 km in the CNGS beam. Exploiting its unique capability to
consistent with the expected number of oscillation events in the standard three-neutrino
framework, 2.30 ± 0.46 (2.21 ± 0.44), for the normal (inverted) mass hierarchy and 0.23
± 0.05 background events.
In this paper we present limits on the existence of a sterile neutrino in the 3+1 neutrino
We thank CERN for the successful operation of the CNGS facility and INFN for the
continuous support given to the experiment through its LNGS laboratory. We acknowledge
funding from our national agencies: Fonds de la Recherche Scientique-FNRS and
Institut InterUniversitaire des Sciences Nucleaires for Belgium, MoSES for Croatia, CNRS and
IN2P3 for France, BMBF for Germany, INFN for Italy, JSPS (Japan Society for the
Promotion of Science), MEXT (Ministry of Education, Culture, Sports, Science and Technology),
QFPU (Global COE programme of Nagoya University, Quest for Fundamental Principles in
the Universe supported by JSPS and MEXT) and Promotion and Mutual Aid Corporation
for Private Schools of Japan for Japan, SNF, the University of Bern for Switzerland, the
Russian Foundation for Basic Research (grant no. 19-02-00213-a, 12-02-12142 ofim), the
Programs of the Presidium of the Russian Academy of Sciences Neutrino physics and
Experimental and theoretical researches of fundamental interactions connected with work on the
accelerator of CERN, the Programs of Support of Leading Schools (grant no. 3110.2014.2),
and the Ministry of Education and Science of the Russian Federation for Russia and the
National Research Foundation of Korea Grant (NRF-2013R1A1A2061654) for Korea. We
are also indebted to INFN for providing fellowships and grants to non-Italian researchers.
We thank the IN2P3 Computing Centre (CC-IN2P3) for providing computing resources for
the analysis and hosting the central database for the OPERA experiment. We are indebted
to our technical collaborators for the excellent quality of their work over many years of
design, prototyping, construction and running of the detector and of its facilities. We also
want to thank Carlo Giunti, Francesco Vissani and Enrico Nardi for fruitful discussions.
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any medium, provided the original author(s) and source are credited.
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M. Vladymyrov,m L. Votano,ab J. L. Vuilleumier,e G. Wilquet,af
B. Wonsak,l C. S. Yoon,z S. Zemskovah
a INR - Institute for Nuclear Research of the Russian Academy of Sciences, RUS-117312 Moscow,
b INFN Sezione di Napoli, 80125 Napoli, Italy
c SINP MSU - Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University,
RUS-119991 Moscow, Russia
d Kobe University, J-657-8501 Kobe, Japan
e Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics (LHEP),
University of Bern, CH-3012 Bern, Switzerland
f METU - Middle East Technical University, TR-06531 Ankara, Turkey
g INFN Sezione di Padova, I-35131 Padova, Italy
h JINR - Joint Institute for Nuclear Research, RUS-141980 Dubna, Russia
i Dipartimento di Fisica dell’Universit`a di Salerno and “Gruppo Collegato” INFN, I-84084 Fisciano
j Dipartimento di Fisica e Astronomia dell’Universit`a di Padova, I-35131 Padova, Italy
k Dipartimento di Fisica dell’Universit`a Federico II di Napoli, I-80125 Napoli, Italy
l Hamburg University, D-22761 Hamburg, Germany
m LPI - Lebedev Physical Institute of the Russian Academy of Sciences, RUS-119991 Moscow, Russia
n INFN - Laboratori Nazionali del Gran Sasso, I-67010 Assergi (L’Aquila), Italy
o Dipartimento di Fisica dell’Universit`a di Bari, I-70126 Bari, Italy
p INFN Sezione di Bari, I-70126 Bari, Italy
q LAPP, Universit´e Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France
r INFN Sezione di Bologna, I-40127 Bologna, Italy
1Now at Kyungpook National University, Daegu, Korea.
2Now at CERN, Geneva, Switzerland.
3Now at Samsung Changwon Hospital, SKKU, Changwon, Korea.
u Dipartimento di Fisica e Astronomia dell’Universit`a di Bologna, I-40127 Bologna, Italy
w INFN Sezione di Roma, I-00185 Roma, Italy
y IRB - Rudjer Boskovic Institute, HR-10002 Zagreb, Croatia
z Gyeongsang National University, 900 Gazwa-dong, Jinju 660-701, Korea
aa Aichi University of Education, J-448-8542 Kariya (Aichi-Ken), Japan
ab INFN - Laboratori Nazionali di Frascati dell’INFN, I-00044 Frascati (Roma), Italy
ac Dipartimento di Fisica dell’Universit`a di Roma “La Sapienza”, I-00185 Roma, Italy
ae Dipartimento di Fisica dell’Universit`a di Milano-Bicocca, I-20126 Milano, Italy
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