arXiv:1311.4750v2 [hep-ex] 16 Apr 2014

Observation of Electron Neutrino Appearance in a Muon Neutrino Beam
K. Abe,46 J. Adam,32 H. Aihara,45, 23 T. Akiri,9 C. Andreopoulos,44 S. Aoki,24 A. Ariga,2 T. Ariga,2 S. Assylbekov,8
D. Autiero,29 M. Barbi,39 G.J. Barker,54 G. Barr,35 M. Bass,8 M. Batkiewicz,13 F. Bay,11 S.W. Bentham,26
V. Berardi,18 B.E. Berger,8 S. Berkman,4 I. Bertram,26 S. Bhadra,58 F.d.M. Blaszczyk,28 A. Blondel,12
C. Bojechko,51 S. Bordoni,15 S.B. Boyd,54 D. Brailsford,17 A. Bravar,12 C. Bronner,25 N. Buchanan,8R.G. Calland,27 J. Caravaca Rodrı́guez,15 S.L. Cartwright,42 R. Castillo,15 M.G. Catanesi,18 A. Cervera,16
D. Cherdack,8 G. Christodoulou,27 A. Clifton,8 J. Coleman,27 S.J. Coleman,7 G. Collazuol,20 K. Connolly,55
L. Cremonesi,38 A. Dabrowska,13 I. Danko,37 R. Das,8 S. Davis,55 P. de Perio,49 G. De Rosa,19 T. Dealtry,44, 35
S.R. Dennis,54, 44 C. Densham,44 F. Di Lodovico,38 S. Di Luise,11 O. Drapier,10 T. Duboyski,38 K. Duffy,35
F. Dufour,12 J. Dumarchez,36 S. Dytman,37 M. Dziewiecki,53 S. Emery,6 A. Ereditato,2 L. Escudero,16 A.J. Finch,26
L. Floetotto,41 M. Friend,14, ∗ Y. Fujii,14, ∗ Y. Fukuda,30 A.P. Furmanski,54 V. Galymov,6 A. Gaudin,51
S. Giffin,39 C. Giganti,36 K. Gilje,32 D. Goeldi,2 T. Golan,57 J.J. Gomez-Cadenas,16 M. Gonin,10 N. Grant,26
D. Gudin,22 D.R. Hadley,54 A. Haesler,12 M.D. Haigh,54 P. Hamilton,17 D. Hansen,37 T. Hara,24 M. Hartz,23, 50
T. Hasegawa,14, ∗ N.C. Hastings,39 Y. Hayato,46 C. Hearty,4, † R.L. Helmer,50 M. Hierholzer,2 J. Hignight,32
A. Hillairet,51 A. Himmel,9 T. Hiraki,25 S. Hirota,25 J. Holeczek,43 S. Horikawa,11 K. Huang,25 A.K. Ichikawa,25
K. Ieki,25 M. Ieva,15 M. Ikeda,46 J. Imber,32 J. Insler,28 T.J. Irvine,47 T. Ishida,14, ∗ T. Ishii,14, ∗ S.J. Ives,17
K. Iyogi,46 A. Izmaylov,16, 22 A. Jacob,35 B. Jamieson,56 R.A. Johnson,7 J.H. Jo,32 P. Jonsson,17 C.K. Jung,32, ‡A.C. Kaboth,17 T. Kajita,47, ‡ H. Kakuno,48 J. Kameda,46 Y. Kanazawa,45 D. Karlen,51, 50 I. Karpikov,22
E. Kearns,3, 23, ‡ M. Khabibullin,22 A. Khotjantsev,22 D. Kielczewska,52 T. Kikawa,25 A. Kilinski,31 J. Kim,4J. Kisiel,43 P. Kitching,1 T. Kobayashi,14, ∗ L. Koch,41 A. Kolaceke,39 A. Konaka,50 L.L. Kormos,26 A. Korzenev,12K. Koseki,14, ∗ Y. Koshio,33, ‡ I. Kreslo,2 W. Kropp,5 H. Kubo,25 Y. Kudenko,22, § S. Kumaratunga,50 R. Kurjata,53
T. Kutter,28 J. Lagoda,31 K. Laihem,41 I. Lamont,26 M. Laveder,20 M. Lawe,42 M. Lazos,27 K.P. Lee,47 C. Licciardi,39
T. Lindner,50 C. Lister,54 R.P. Litchfield,54 A. Longhin,20 L. Ludovici,21 M. Macaire,6 L. Magaletti,18 K. Mahn,50
M. Malek,17 S. Manly,40 A.D. Marino,7 J. Marteau,29 J.F. Martin,49 T. Maruyama,14, ∗ J. Marzec,53 E.L. Mathie,39
V. Matveev,22 K. Mavrokoridis,27 E. Mazzucato,6 M. McCarthy,4 N. McCauley,27 K.S. McFarland,40 C. McGrew,32
C. Metelko,27 M. Mezzetto,20 P. Mijakowski,31 C.A. Miller,50 A. Minamino,25 O. Mineev,22 S. Mine,5 A. Missert,7M. Miura,46, ‡ L. Monfregola,16 S. Moriyama,46, ‡ Th.A. Mueller,10 A. Murakami,25 M. Murdoch,27 S. Murphy,11J. Myslik,51 T. Nagasaki,25 T. Nakadaira,14, ∗ M. Nakahata,46, 23 T. Nakai,34 K. Nakamura,23, 14, ∗ S. Nakayama,46, ‡T. Nakaya,25, 23 K. Nakayoshi,14, ∗ D. Naples,37 C. Nielsen,4 M. Nirkko,2 K. Nishikawa,14, ∗ Y. Nishimura,47
H.M. O’Keeffe,26 R. Ohta,14, ∗ K. Okumura,47, 23 T. Okusawa,34 W. Oryszczak,52 S.M. Oser,4 R.A. Owen,38
Y. Oyama,14, ∗ V. Palladino,19 V. Paolone,37 D. Payne,27 G.F. Pearce,44 O. Perevozchikov,28 J.D. Perkin,42
Y. Petrov,4 L.J. Pickard,42 E.S. Pinzon Guerra,58 C. Pistillo,2 P. Plonski,53 E. Poplawska,38 B. Popov,36, ¶M. Posiadala,52 J.-M. Poutissou,50 R. Poutissou,50 P. Przewlocki,31 B. Quilain,10 E. Radicioni,18 P.N. Ratoff,26
M. Ravonel,12 M.A.M. Rayner,12 A. Redij,2 M. Reeves,26 E. Reinherz-Aronis,8 F. Retiere,50 A. Robert,36
P.A. Rodrigues,40 P. Rojas,8 E. Rondio,31 S. Roth,41 A. Rubbia,11 D. Ruterbories,8 R. Sacco,38 K. Sakashita,14, ∗F. Sánchez,15 F. Sato,14 E. Scantamburlo,12 K. Scholberg,9, ‡ J. Schwehr,8 M. Scott,50 Y. Seiya,34 T. Sekiguchi,14, ∗H. Sekiya,46, ‡ D. Sgalaberna,11 M. Shiozawa,46, 23 S. Short,17 Y. Shustrov,22 P. Sinclair,17 B. Smith,17 R.J. Smith,35
M. Smy,5 J.T. Sobczyk,57 H. Sobel,5, 23 M. Sorel,16 L. Southwell,26 P. Stamoulis,16 J. Steinmann,41 B. Still,38
Y. Suda,45 A. Suzuki,24 K. Suzuki,25 S.Y. Suzuki,14, ∗ Y. Suzuki,46, 23 T. Szeglowski,43 R. Tacik,39, 50 M. Tada,14, ∗S. Takahashi,25 A. Takeda,46 Y. Takeuchi,24, 23 H.K. Tanaka,46, ‡ H.A. Tanaka,4, † M.M. Tanaka,14, ∗ D. Terhorst,41
R. Terri,38 L.F. Thompson,42 A. Thorley,27 S. Tobayama,4 W. Toki,8 T. Tomura,46 Y. Totsuka,∗∗ C. Touramanis,27
T. Tsukamoto,14, ∗ M. Tzanov,28 Y. Uchida,17 K. Ueno,46 A. Vacheret,35 M. Vagins,23, 5 G. Vasseur,6 T. Wachala,13
A.V. Waldron,35 C.W. Walter,9, ‡ D. Wark,44, 17 M.O. Wascko,17 A. Weber,44, 35 R. Wendell,46, ‡ R.J. Wilkes,55
M.J. Wilking,50 C. Wilkinson,42 Z. Williamson,35 J.R. Wilson,38 R.J. Wilson,8 T. Wongjirad,9 Y. Yamada,14, ∗K. Yamamoto,34 C. Yanagisawa,32, †† S. Yen,50 N. Yershov,22 M. Yokoyama,45, ‡ T. Yuan,7 A. Zalewska,13J. Zalipska,31 L. Zambelli,36 K. Zaremba,53 M. Ziembicki,53 E.D. Zimmerman,7 M. Zito,6 and J. Żmuda57(The T2K Collaboration)
1

University of Alberta, Centre for Particle Physics, Department of Physics, Edmonton, Alberta, Canada
2
University of Bern, Albert Einstein Center for Fundamental Physics,
Laboratory for High Energy Physics (LHEP), Bern, Switzerland
3
Boston University, Department of Physics, Boston, Massachusetts, U.S.A.

2
4

University of British Columbia, Department of Physics and Astronomy, Vancouver, British Columbia, Canada
5
University of California, Irvine, Department of Physics and Astronomy, Irvine, California, U.S.A.
6
IRFU, CEA Saclay, Gif-sur-Yvette, France
7
University of Colorado at Boulder, Department of Physics, Boulder, Colorado, U.S.A.
8
Colorado State University, Department of Physics, Fort Collins, Colorado, U.S.A.
9
Duke University, Department of Physics, Durham, North Carolina, U.S.A.
10
Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, Palaiseau, France
11
ETH Zurich, Institute for Particle Physics, Zurich, Switzerland
12
University of Geneva, Section de Physique, DPNC, Geneva, Switzerland
13
H. Niewodniczanski Institute of Nuclear Physics PAN, Cracow, Poland
14
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan
15
Institut de Fisica d’Altes Energies (IFAE), Bellaterra (Barcelona), Spain
16
IFIC (CSIC & University of Valencia), Valencia, Spain
17
Imperial College London, Department of Physics, London, United Kingdom
18
INFN Sezione di Bari and Università e Politecnico di Bari, Dipartimento Interuniversitario di Fisica, Bari, Italy
19
INFN Sezione di Napoli and Università di Napoli, Dipartimento di Fisica, Napoli, Italy
20
INFN Sezione di Padova and Università di Padova, Dipartimento di Fisica, Padova, Italy
21
INFN Sezione di Roma and Università di Roma “La Sapienza”, Roma, Italy
22
Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
23
Kavli Institute for the Physics and Mathematics of the Universe (WPI),
Todai Institutes for Advanced Study, University of Tokyo, Kashiwa, Chiba, Japan
24
Kobe University, Kobe, Japan
25
Kyoto University, Department of Physics, Kyoto, Japan
26
Lancaster University, Physics Department, Lancaster, United Kingdom
27
University of Liverpool, Department of Physics, Liverpool, United Kingdom
28
Louisiana State University, Department of Physics and Astronomy, Baton Rouge, Louisiana, U.S.A.
29
Université de Lyon, Université Claude Bernard Lyon 1, IPN Lyon (IN2P3), Villeurbanne, France
30
Miyagi University of Education, Department of Physics, Sendai, Japan
31
National Centre for Nuclear Research, Warsaw, Poland
32
State University of New York at Stony Brook, Department of Physics and Astronomy, Stony Brook, New York, U.S.A.
33
Okayama University, Department of Physics, Okayama, Japan
34
Osaka City University, Department of Physics, Osaka, Japan
35
Oxford University, Department of Physics, Oxford, United Kingdom
36
UPMC, Université Paris Diderot, CNRS/IN2P3, Laboratoire dePhysique Nucléaire et de Hautes Energies (LPNHE), Paris, France
37
University of Pittsburgh, Department of Physics and Astronomy, Pittsburgh, Pennsylvania, U.S.A.
38
Queen Mary University of London, School of Physics and Astronomy, London, United Kingdom
39
University of Regina, Department of Physics, Regina, Saskatchewan, Canada
40
University of Rochester, Department of Physics and Astronomy, Rochester, New York, U.S.A.
41
RWTH Aachen University, III. Physikalisches Institut, Aachen, Germany
42
University of Sheffield, Department of Physics and Astronomy, Sheffield, United Kingdom
43
University of Silesia, Institute of Physics, Katowice, Poland
44
STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington, United Kingdom
45
University of Tokyo, Department of Physics, Tokyo, Japan
46
University of Tokyo, Institute for Cosmic Ray Research, Kamioka Observatory, Kamioka, Japan
47
University of Tokyo, Institute for Cosmic Ray Research, Research Center for Cosmic Neutrinos, Kashiwa, Japan
48
Tokyo Metropolitan University, Department of Physics, Tokyo, Japan
49
University of Toronto, Department of Physics, Toronto, Ontario, Canada
50
TRIUMF, Vancouver, British Columbia, Canada
51
University of Victoria, Department of Physics and Astronomy, Victoria, British Columbia, Canada
52
University of Warsaw, Faculty of Physics, Warsaw, Poland
53
Warsaw University of Technology, Institute of Radioelectronics, Warsaw, Poland
54
University of Warwick, Department of Physics, Coventry, United Kingdom
55
University of Washington, Department of Physics, Seattle, Washington, U.S.A.
56
University of Winnipeg, Department of Physics, Winnipeg, Manitoba, Canada
57
Wroclaw University, Faculty of Physics and Astronomy, Wroclaw, Poland
58
York University, Department of Physics and Astronomy, Toronto, Ontario, Canada(Dated: April 17, 2014)
The T2K experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28
electron neutrino events were detected with an energy distribution consistent with an appearance
signal, corresponding to a significance of 7.3σ when compared to 4.92 ± 0.55 expected background
events. In the PMNS mixing model, the electron neutrino appearance signal depends on several

3parameters including three mixing angles θ12 , θ23 , θ13 , a mass difference ∆m232 and a CP violatingphase δCP . In this neutrino oscillation scenario, assuming |∆m232 | = 2.4 × 10−3 eV2 , sin2 θ23 = 0.5,
+0.045and ∆m232 > 0 (∆m232 < 0), a best-fit value of sin2 2θ13 = 0.140+0.038−0.032 (0.170−0.037 ) is obtained at
δCP = 0. When combining the result with the current best knowledge of oscillation parameters
including the world average value of θ13 from reactor experiments, some values of δCP are disfavoredat the 90% CL.
PACS numbers: 14.60.Pq, 14.60.Lm, 25.30.Pt, 29.40.Ka

Introduction—The discovery of neutrino oscillations
using atmospheric neutrinos was made by SuperKamiokande in 1998 [1]. Since then, many other experiments have confirmed the phenomenon of neutrino oscillations through various disappearance modes of flavor
transformations. However, to date, there has not been an
observation of the explicit appearance of a different neutrino flavor from neutrinos of another flavor through neutrino oscillations. In 2011, the T2K collaboration published the first indication of electron neutrino appearance
from a muon neutrino beam at 2.5σ significance based ona data set corresponding to 1.43 × 1020 protons on target(POT) [2, 3]. This result was followed by the publicationof further evidence for electron neutrino appearance at
3.1σ in early 2013 [4]. This paper presents new resultsfrom the T2K experiment that establish, at greater than
5σ, the observation of electron-neutrino appearance froma muon-neutrino beam.
In a three-flavor framework, neutrino oscillations aredescribed by the PMNS matrix [5, 6] which is parameterized by three mixing angles θ12 , θ23 , θ13 , and a CP
violating phase δCP . In this framework the probability
for νµ → νe oscillation can be expressed [7] as
P (νµ → νe ) ≃ sin2 θ23 sin2 2θ13 sin2

∆m231 L4E

sin 2θ12 sin 2θ23
∆m221 L∆m231 Lsin
sin2 2θ13 sin2
sin δCP2 sin θ13
4E
4E
+ (CP even term, solar term, matter effect term),(1)−where L is the neutrino propagation distance and E is
the neutrino energy. The measurement of νµ → νe oscillations is of particular interest because this mode is sensitive to both θ13 and δCP . The first indication of non-zeroθ13 was published by T2K [3] based on the measurementof νµ → νe oscillations. More recently, indications ofνµ → νe oscillations were also reported by the MINOS
experiment [12]. The value of θ13 is now precisely known
to be 9.1 ± 0.6◦ from measurements of ν e disappearance
in reactor neutrino experiments [8–11]. Using the reactor measurement of θ13 , the νµ → νe appearance modecan be used to explore CP violation, which has yet tobe observed in the lepton sector. CP violation, as shownin Equation 1, is governed by the second term and canbe as large as 27% of the first term for the T2K experimental setup when using current values of the neutrino
oscillation parameters.
T2K Experiment—T2K operates at the J-PARC facility in Tokai, Japan. A muon neutrino beam is produced

from the decay of charged pions and kaons generated by
30 GeV protons hitting a graphite target and focusedby three magnetic horns. Downstream of the horns isthe decay volume, 96 meters in length, followed by thebeam dump and muon monitors (MUMON). The neutrino beam illuminates an on-axis detector and off-axis
detectors positioned at an angle of 2.5◦ relative to thebeam direction. The resulting energy spectrum, peaked
at 0.6 GeV for the off-axis detectors, reduces the νe contamination and the feed-down backgrounds to the νeappearance signal from higher energy neutrinos. The
near detector complex at 280 meters from the target
is used to measure the neutrino beam direction, spectrum, and composition before oscillations and to measure neutrino cross sections. The complex consists of an
on-axis detector (INGRID) and a suite of off-axis detectors (ND280) that reside within a 0.2 T magnet [2]. The
Super-Kamiokande (SK) 50 kt water Cherenkov detector, situated 295 km away, is used to detect the oscillated
neutrinos.
The results presented here are based on data takenfrom January 2010 to May 2013. During this period theproton beam power has steadily increased and reached
220 kW continuous operation with a world record of
1.2 × 1014 protons per pulse. The total neutrino beam
exposure at SK corresponds to 6.57 × 1020 POT.
Neutrino Beam Flux—The neutrino beam flux [13]is predicted by modeling interactions of the primarybeam protons in a graphite target using external hadronproduction data from the CERN NA61/SHINE experiment [14, 15] and the FLUKA2008 package [16, 17].
GEANT3 [18] with GCALOR [19] simulates propagationof the secondary/tertiary pions and kaons, and their decays into neutrinos. The νe component (including a small
amount of ν e ) in the beam is estimated to be less than
1% of the flux below 1.5 GeV, and constitutes an irreducible background to the νe appearance search. This
component is generated predominantly by the decay ofmuons for Eν < 1 GeV and by kaons for Eν > 1 GeV.
The neutrino flux uncertainties are dominated by thehadron production uncertainties, with contributions fromthe neutrino beam direction and the proton beam uncertainties. The neutrino beam direction, monitored indirectly by MUMON on a spill-by-spill basis, and directly
by INGRID [20], was found to be well within the required±1 mrad during the full run period. INGRID also measured the neutrino interaction rate per POT to be stable

Data/MC

within 0.7%. The total systematic error for the absolute
flux prediction is evaluated to be 10–15% in the relevantenergy range. Furthermore, the uncertainty on the ratioof the flux predictions at the far and the near detectors
is less than 2% around the peak.
Neutrino Interaction Simulations and Cross Section
Parameters—The NEUT neutrino interaction generator [21] is used to simulate neutrino interactions in the
INGRID, ND280, and SK detectors. At interaction energies typical of the T2K beam, the dominant charged current (CC) interactions are charged current quasi-elastic(CCQE) and single resonant pion production. The crosssection parameterization can be divided into two categories: parameters common to interactions at both
ND280 and SK, and parameters evaluated separately forthe two detectors. Parameters in the first category comprise the axial masses for CCQE (MAQE ) and single resonant pion production (MARES ), and normalizations for
CCQE, CC single pion, and neutral current (NC) 1π 0interactions. Parameters in the second category are typically related to the interaction target—primarily carbon
at ND280 and oxygen at SK—and include Fermi momentum, binding energy, and spectral function modelingfor the CCQE nuclear model. Also in this category arenormalizations for other CC and NC cross sections, theνe /νµ CC cross section ratio, pion production parameters, and final state interactions of pions exiting the nucleus. External data sets, primarily from [22–24], areused to determine the initial values and prior uncertainties of the parameters [4].
ND280 Measurements, Flux and Common Cross Section fits—The energy spectrum of the neutrino beam andthe neutrino cross section parameters are constrained using νµ CC interactions in ND280. The fine-grained detectors (FGDs) [25] are scintillator trackers that serveas the primary neutrino target, and the momentum andidentity of the particles emerging from the interaction aredetermined by the time projection chambers (TPCs) [26]interleaved with the FGDs. The muon is assumed tobe the highest-momentum, negative-curvature track thatemerges from the FGD fiducial volume with an energy deposition consistent with a muon in the TPC downstreamof the FGD. Tracks found in the TPC upstream of the
FGD are used to veto external background events.
The ND280 analysis includes many improvements overthe previous T2K electron neutrino appearance measurement [4]. Candidate events are now divided into threesamples: CC-0π, dominated by CCQE interactions; CC1π + , dominated by CC resonant pion production; andCC-other. The samples are defined by the number of pions in the observed final state. A π + can be identified
in one of three ways: an FGD+TPC track with positive
curvature and a TPC charge deposition consistent witha pion, an FGD-contained track with a charge deposition
consistent with a pion, or a delayed energy deposit due
to a decay electron from stopped π + → µ+ in the FGD.

Number of events

4

2500

Prediction before ND constraint

2000

Prediction after ND constraint

1500

Data

1000
500
0
1.2
1
0.8
0

Muon momentum (MeV/c)
500

1000

1500

2000

2500

>3000

Muon momentum (MeV/c)

FIG. 1. The muon momentum distribution for the ND280
CC-0π sample (upper). The black points represent the data,
the blue histogram shows the MC prediction before data constraint, and the red histogram shows the MC prediction after
constraint. The lower plot shows the ratio of data to MC for
the pre- and post-constraint cases.

To tag a π − , only negative curvature FGD+TPC tracks
are used. A π 0 is identified if there exists a track in the
TPC with a charge deposition consistent with an electronfrom a γ conversion. Events containing no pions are classified as CC-0π, events with exactly one π + and no π −or π 0 are classified as CC-1π + , and all other CC eventsare classified as CC-other. There are 17369, 4047, and4173 data events in the CC-0π, CC-1π + , and CC-othersamples, respectively. The ND280 data set used for thisanalysis corresponds to 5.90 × 1020 POT.
The three samples are fit with 25 beam flux parameters
at ND280 (11 Eνµ , 5 Eν̄µ , 7 Eνe , and 2 Eν̄e bins), 21 crosssection parameters (5 in common with SK, and 16 used
only for ND280), as well as 210 parameters describing theND280 detector systematics (10 momentum × 7 angle
bins for each sample). The dominant detector uncertainties come from events occurring outside the FGD fiducial
volume and from pion reinteractions in the detector. The
ND280 measurements constrain the SK flux parameters
due to the flux covariance derived from beam simulations.
The predicted numbers of ND280 events in Monte Carlo(MC), using the best-fit parameters, are 17352, 4110, and4119 for the CC-0π, CC-1π + , and CC-other samples, respectively. A χ2 goodness-of-fit test returns a p-value of0.66, indicating no disagreement between the data andthe prediction using best-fit parameters. Figure 1 shows
the muon momentum distribution of the CC-0π sample,
and the improvement in data and MC agreement whenusing the best-fit parameters.
The fit to the ND280 data gives estimates for 22 beam
flux parameters at SK, the 5 common cross section parameters, and their covariance. Using the ND280 information reduces the uncertainty on the expected numberof electron-like events at SK due to the propagated parameters from 25.9% to 2.9%.
SK Measurements—The SK detector is composed of

5Eνrec =

m2p − (mn − Eb )2 − m2e + 2(mn − Eb )Ee,2(mn − Eb − Ee + pe cos θe )(2)where mn (mp ) is the neutron (proton) mass, Eb is theneutron binding energy in oxygen (27 MeV), me is theelectron mass, Ee is its energy, and θe is the angle of theelectron direction relative to the beam direction.
The final selection criterion removes additional π 0
background events using a new reconstruction algorithm,
based on an extension of the model described in Reference [27], to determine the kinematics of all final stateparticles. The new algorithm is a maximum-likelihood
fit in which charge and time probability density functions (PDFs) are constructed for every PMT hit for agiven particle hypothesis with a set of 7 parameters:
the vertex position, the timing, the direction and themomentum. Multiple-particle fit hypotheses are constructed by summing the charge contributions from eachconstituent particle. Different neutrino final states are
distinguished by comparing the best-fit likelihood resulting from the fit of each hypothesis. To separate π 0
events from νe CC events, both the reconstructed π 0mass (mπ0 ) and the ratio of the best-fit likelihoods ofthe π 0 and electron fits (Lπ0 /Le ) are used. Figure 2
shows the ln(Lπ0 /Le ) vs π 0 mass distribution for signal
νe -CC events and events containing a π 0 in the MC sample, as well as the rejection cut line. Events that satisfyln(Lπ0 /Le ) < 175 − 0.875 × mπ0 (MeV/c2 ) constitutethe final νe candidate sample. This cut removes 69% of

the π 0 background events relative to the previous T2K
νe appearance selection, with only a 2% loss in signal
efficiency [3].

πln(L 0/Le)

an inner detector (ID) and an outer detector (OD). The
ID has a water fiducial volume (FV) of 22.5 kt that is
equipped with 11129 photomultiplier tubes (PMT) andis surrounded by the 2 m wide OD. Neutrino events at
SK are selected if the Cherenkov ring is consistent withan energy above 30 MeV in the ID with low activity
in the OD to reject any entering background or exiting
events. These events are labeled fully-contained (FC).
The FC fiducial volume (FCFV) sample is obtained byapplying the further cut that the event vertex is at least
2 m away from the ID tank wall. A timing cut of −2 to10 µs relative to the first beam bunch arrival is applied to
distinguish T2K data from other neutrino samples suchas atmospheric neutrino interactions. The timing cut
reduces the contamination from other neutrino sources
to 0.0085 events in the full sample.
To select νe interaction candidate events in the FCFV
sample, a single electron-like Cherenkov ring is required.
The reconstructed electron momentum (pe ) is required
to exceed 100 MeV/c to eliminate decay-electrons fromstopping muons generated by CC interactions and pions in NC interactions. In addition, events are required
to have a reconstructed neutrino energy (Eνrec ) below
1250 MeV. Nearly all of the oscillated νe signal events
are below this value, while most of the intrinsic beam
νe background events have higher energies. The Eνrec is
calculated assuming a CCQE interaction as

400
350
300
250

νe Signal
π0 Background

200
150
100
50
0
0

50

100

150

200

250

π0 Mass (MeV/c2)

FIG. 2. The ln(Lπ0 /Le ) vs mπ0 distribution is shown for bothsignal νe -CC events (boxes) and background events containinga π 0 (blue scale). The red line indicates the location of the π 0rejection cut. Events in the upper right corner are rejected.

A summary of the number of events passing each selection cut is shown in Table I. After all cuts, the total number of candidate νe events selected in data is 28,
which is significantly larger than the 4.92±0.55 expected
events for θ13 = 0. For sin2 2θ13 = 0.1 and δCP = 0, theexpected number is 21.6, as shown in Table I.
TABLE I. The expected number of signal and background
events passing each selection stage assuming sin2 2θ13 = 0.1,
sin2 θ23 = 0.5, |∆m232 | = 2.4 × 10−3 eV2 , δCP = 0, and∆m232 > 0, compared to the observed number in data. Interactions in the true FV are based on the MC truth information while all other numbers are based on the reconstructed
information and have been rounded off after addition to avoid
rounding error.
Totalνµ →νe νµ +ν µ νe +ν e
NC
CC
CC
CC
MC
Interactions in FV 27.1 325.7 16.0 288.1 656.8
FCFV
377 26.2 247.8 15.4 83.0 372.4+Single-ring
193 22.7 142.4
9.8 23.5 198.4
22.4
5.6
9.7 16.3 54.2
+e-like PID
60+pe >100MeV/c
57
22.0
3.7
9.7 14.0 49.4
19.6
0.7
7.9 11.8 40.0
+No decay-e
44+Eνrec <1250MeV
39
18.8
0.2
3.7
9.0 31.7+Non-π 0 -like
28
17.3
0.1
3.2
1.0 21.6
Selection

Data

The systematic uncertainty due to the SK selection
cuts is evaluated using various data and MC samples.
The uncertainty for both the FC and the FV selection
is 1%. The decay-electron rejection cut has errors of
0.2-0.4%, depending on neutrino flavor and interaction
type. The uncertainties for the single electron-like ring
selection and π 0 rejection are estimated by using the SK

6

L(Nobs , ~x, ~o, f~) = Lnorm (Nobs ; ~o, f~) × Lshape (~x; ~o, f~)×Lsyst (f~) × Lconst (~o),(3)
where Nobs is the number of observed events, ~x is a set ofkinematic variables, ~o represents oscillation parameters,
and f~ describes systematic uncertainties. In the fit, thelikelihood is integrated over the nuisance parameters to
obtain a marginalized likelihood for the parameters ofinterest.
Lnorm is calculated from a Poisson distribution using the mean value from the predicted number of MC
events. Lsyst (f~) constrains the 27 systematic parametersfrom the ND280 fit, the SK-only cross section parameters, and the SK selection efficiencies. Table II shows
the uncertainties on the predicted number of signal νeevents. The Lshape term uses x=(pe , θe ) to distinguishthe νe signal from backgrounds. An alternative analysis
uses x = Eνrec , the reconstructed neutrino energy. In order to combine the results presented in this letter withother measurements to better constrain sin2 2θ13 and δCP ,the Lconst term can also be used to apply additional constraints on sin2 2θ13 , sin2 θ23 and ∆m232 .

TABLE II. The uncertainty (RMS/mean in %) on the predicted number of signal νe events for each group of systematic
uncertainties for sin2 2θ13 = 0.1 and 0. The uncorrelated νinteraction uncertainties are those coming from parts of theneutrino interaction model that cannot be constrained with
ND280.
Error source [%]sin2 2θ13 = 0.1 sin2 2θ13 = 0
Beam flux and near detector
2.9
4.8(w/o ND280 constraint)(25.9)(21.7)
Uncorrelated ν interaction
7.5
6.8
Far detector and FSI+SI+PN
3.5
7.3
Total
8.8
11.1

Data
Best fit
Background component

10
5

Angle (degrees)

atmospheric neutrino data and SK cosmic-ray muons.
Electron-neutrino CC-enriched control samples based onthese cuts were prepared, and the differences between
MC predictions and data are used to extract the systematic uncertainty. The uncertainty associated with theπ 0 background is determined by constructing a hybrid
sample with either an electron-like ring taken from theatmospheric data sample or from decay-electrons selectedin the stopping muon data sample, and a MC-generated
gamma ray assuming π 0 kinematics. The selection cut
systematic uncertainty is calculated to be 1.6% for signal
events and 7.3% for background events. The total SK
selection uncertainty is 2.1% for the νe candidate events
assuming sin2 2θ13 = 0.1.
Additional SK systematic uncertainties are due to
final-state interactions (FSI) of pions that occur inside
the target nucleus, as well as secondary interactions (SI)of pions and photo-nuclear (PN) interactions of photons
that occur outside of the target nucleus. The treatmentof the FSI and SI uncertainties is the same as in the previous analysis [28]. For this analysis, a new simulation of
PN interactions has been added to the SK MC. In the final νe event sample, 15% of the remaining π 0 backgroundis due to events where one of the π 0 decay photons is absorbed in a PN interaction. A systematic uncertainty of
100% is assumed for the normalization of the PN crosssection.
Oscillation Analysis—The neutrino oscillation parameters are evaluated using a binned extended maximumlikelihood fit. The likelihood consists of four components:a normalization term (Lnorm ), a term for the spectrum
shape (Lshape ), a systematics term (Lsyst ), and a constraint term (Lconst ) from other measurements,

180
1

150

Data
Best fit

120

0.8

90

0.6

60

0.4

30

0.2

0

5

0

0
500
1000
1500
Momentum (MeV/c)

FIG. 3. The (pe , θe ) distribution for νe candidate events with
the MC prediction using the primary method best-fit value ofsin2 2θ13 = 0.140 (normal hierarchy).

The following oscillation parameters are fixed in theanalysis: sin2 θ12 = 0.306, ∆m221 = 7.6 × 10−5 eV2 [29],sin2 θ23 = 0.5, |∆m232 | = 2.4 × 10−3 eV2 [30] and δCP = 0.
For the normal (inverted) hierarchy case, the best-fit
value with a 68% confidence level (CL) is sin2 2θ13 =+0.0450.140+0.038−0.032 (0.170−0.037 ). Figure 3 shows the best-fit result, with the 28 observed νe events. The alternative
analysis using Eνrec and a profile likelihood method produces consistent best-fit values and nearly identical confidence regions. Figure 4 shows the Eνrec distribution with
the MC prediction for the best-fit θ13 value in the alternative analysis.
The significance for a non-zero θ13 is calculated to be
7.3σ, using the difference of log likelihood values between
the best-fit θ13 value and θ13 = 0. An alternative methodof calculating the significance, by generating a large number of toy MC experiments assuming θ13 = 0, also returns
a value of 7.3σ. These significances were calculated using a test statistic having fixed values for θ23 and δCP .

Number of νe candidate events

7

8

Data
Best fit
Background component6

Fit region < 1250 MeV

4
2
0

>2000
500
1000
1500
Reconstructed neutrino energy (MeV)0δCP (π)FIG. 4. The Eνrec distribution for νe candidate events with
the MC prediction at the best fit of sin2 2θ13 = 0.144 (normal
hierarchy) by the alternative binned Eνrec analysis.

1
0.5∆m232>0-0.5

68% CL
90% CLBest fit
PDG2012 1σ rangeδCP (π)

0

-1
1

0.5
0

-0.5
-1
0

∆m232<00.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

sin22θ13

FIG. 5. The 68% and 90% CL allowed regions for sin2 2θ13 ,as a function of δCP assuming normal hierarchy (top) andinverted hierarchy (bottom). The solid line represents thebest fit sin2 2θ13 value for given δCP values. The values ofsin2 θ23 and ∆m232 are varied in the fit with the constraint
from [30]. The shaded region shows the average θ13 value
from the PDG2012 [8].

For any values for these parameters, consistent with theirpresent uncertainties, the significance remains above 7σ.
As the precision of this measurement increases, the uncertainty from other oscillation parameters becomes increasingly important. The uncertainties on θ23 and ∆m232
are taken into account in the fit by adding a Lconst term
and marginalizing the likelihood over θ23 and ∆m232 . The
Lconst term is the likelihood as a function of sin2 θ23 and∆m232 , obtained from the T2K νµ disappearance measurement [30]. The value of δCP and the hierarchy areheld fixed in the fit. Performing the fit for all values ofδCP , the allowed 68% and 90% CL regions for sin2 2θ13
are obtained as shown in Figure 5. For δCP = 0 andnormal (inverted) hierarchy case, the best-fit value with
+0.051
a 68% CL is sin2 2θ13 = 0.136+0.044−0.033 (0.166−0.042 ). With
the current statistics, the correlation between the νµ disappearance and νe appearance measurements in T2K is
negligibly small.
Constraints on δCP are obtained by combining our results with the θ13 value measured by reactor experiments.
The additional likelihood constraint term on sin2 2θ13 isdefined as exp{−(sin2 2θ13 − 0.098)2/(2(0.0132))}, where
0.098 and 0.013 are the averaged value and the error ofsin2 2θ13 from PDG2012 [8]. The −2∆ ln L curve as afunction of δCP is shown in Figure 6, where the likelihood is marginalized over sin2 2θ13 , sin2 θ23 and ∆m232 .
The combined T2K and reactor measurements preferδCP = −π/2. The 90% CL limits shown in Figure 6
are evaluated by using the Feldman-Cousins method [31]in order to extract the excluded region. The data excludes δCP between 0.19π and 0.80π (−π and −0.97π,
and −0.04π and π) with normal (inverted) hierarchy at
90% CL.
The maximum value of −2∆ ln L is 3.38 (5.76) at
δCP = π/2 for normal (inverted) hierarchy case. Thisvalue is compared with a large number of toy MC experiments, generated assuming δCP = −π/2, sin2 2θ13 = 0.1,
sin2 θ23 = 0.5 and ∆m232 = 2.4 × 10−3 eV2 . The MC
averaged value of −2∆ ln L at δCP = π/2 is 2.20 (4.10)
for normal (inverted) hierarchy case, and the probability of obtaining a value greater or equal to the observed
value is 34.1% (33.4%). With the same MC settings,
the expected 90% CL exclusion region is evaluated to be
between 0.35π and 0.63π (0.09π and 0.90π) radians fornormal (inverted) hierarchy case.
Conclusions—T2K has made the first observation ofelectron neutrino appearance in a muon neutrino beamwith a peak energy of 0.6 GeV and a baseline of 295 km.
With the fixed parameters |∆m232 | = 2.4 × 10−3 eV2 ,sin2 θ23 = 0.5, δCP = 0, and ∆m232 > 0 (∆m232 < 0), a+0.045best-fit value of sin2 2θ13 = 0.140+0.038−0.032 (0.170−0.037 ) is
obtained, with a significance of 7.3σ over the hypothesisof sin2 2θ13 = 0. When combining the T2K result withthe world average value of θ13 from reactor experiments,
some values of δCP are disfavored at the 90% CL.
T2K will continue to take data to measure the neutrino
oscillation parameters more precisely and to further explore CP violation in the lepton sector.
We thank the J-PARC staff for superb accelerator performance and the CERN NA61 collaboration for providing valuable particle production data. We acknowledge
the support of MEXT, Japan; NSERC, NRC and CFI,
Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Edu-

-2∆lnL8654∆m232>0∆m232<090% CL ( ∆m232>0)
90% CL ( ∆m232<0)

3
2
1
0
-1

-0.5

0

0.5

1

δCP (π)FIG. 6. The −2∆ ln L value as a function of δCP for normal hierarchy (solid line) and inverted hierarchy (dotted line). The
likelihood is marginalized over sin2 2θ13 , sin2 θ23 and ∆m232 .
The solid (dotted) line with markers corresponds to the 90%
CL limits for normal (inverted) hierarchy, evaluated by using
the Feldman-Cousins method. The δCP regions with values
above the lines are excluded at 90% CL.

cation, Poland; RAS, RFBR and MES, Russia; MICINN
and CPAN, Spain; SNSF and SER, Switzerland; STFC,
U.K.; and DOE, U.S.A. We also thank CERN for the
UA1/NOMAD magnet, DESY for the HERA-B magnet
mover system, NII for SINET4, the WestGrid and SciNet
consortia in Compute Canada, and GridPP, UK. In addition participation of individual researchers and institutions has been further supported by funds from: ERC(FP7), EU; JSPS, Japan; Royal Society, UK; DOE Early
Career program, U.S.A.

∗

also at J-PARC, Tokai, Japan
also at Institute of Particle Physics, Canada‡
affiliated member at Kavli IPMU (WPI), the Universityof Tokyo, Japan§
also at Moscow Institute of Physics and Technology andNational Research Nuclear University (MEPhI), Moscow,
Russia¶also at JINR, Dubna, Russia∗∗
deceased††also at BMCC/CUNY, Science Department, New York,New York, U.S.A.
[1] Y. Fukuda et al. (Super-Kamiokande Collaboration),
Phys.Rev.Lett. 81, 1562 (1998).
[2] K.
Abe
et
al.
(T2K
Collaboration),†

Nucl.Instrum.Meth. A659, 106 (2011).
et
al.
(T2K
Collaboration),[3] K.
Abe
Phys.Rev.Lett. 107, 041801 (2011).
[4] K.
Abe
et
al.
(T2K
Collaboration),
Phys.Rev. D88, 032002 (2013).
[5] Z. Maki,
M. Nakagawa,and S. Sakata,Prog.Theor.Phys. 28, 870 (1962).
[6] B. Pontecorvo, Sov.Phys.JETP 26, 984 (1968).
[7] M. Freund, Phys.Rev. D64, 053003 (2001).
[8] J.
Beringeretal.
(Particle
DataGroup),
Phys.Rev. D86, 010001 (2012).
[9] F.
An
et
al.
(Daya
Bay
Collaboration),
Phys.Rev.Lett. 108, 171803 (2012).
[10] J.
Ahn
et
al.
(RENO
collaboration),
Phys.Rev.Lett. 108, 191802 (2012).
[11] Y. Abe et al. (Double Chooz Collaboration),
Phys.Rev.Lett. 108, 131801 (2012).
etal.
(MINOS
Collaboration),[12] P.
AdamsonPhys.Rev.Lett. 110, 171801 (2013).
[13] K.
Abe
et
al.
(T2K
Collaboration),
Phys.Rev. D87, 012001 (2013).
[14] N. Abgrall et al. (NA61/SHINE Collaboration),
Phys.Rev. C84, 034604 (2011).
[15] N. Abgrall et al. (NA61/SHINE Collaboration),
Phys.Rev. C85, 035210 (2012).
[16] A. Ferrari, P. R. Sala, A. Fasso, and J. Ranft, CERN2005-010, SLAC-R-773, INFN-TC-05-11.
[17] G. Battistoni, S. Muraro, P. R. Sala, F. Cerutti, A. Ferrari, et al., AIP Conf.Proc. 896, 31 (2007).
[18] R. Brun, F. Carminati, and S. Giani, CERN-W5013(1994).
[19] C. Zeitnitz and T. Gabriel, In *Corpus Christi 1992,
Calorimetry in high energy physics* 394-404 (1992).
[20] K.
Abe
et
al.
(T2K
Collaboration),
Nucl.Instrum.Meth. A694, 211 (2012).
[21] Y. Hayato, Acta Phys.Polon. B40, 2477 (2009).
[22] A. A. Aguilar-Arevalo et al. (MiniBooNE Collaboration),
Phys.Rev. D83, 052009 (2011).
[23] A. A. Aguilar-Arevalo et al. (MiniBooNE Collaboration),
Phys.Rev. D83, 052007 (2011).
[24] A. A. Aguilar-Arevalo et al. (MiniBooNE Collaboration),
Phys.Rev. D81, 013005 (2010).
[25] P. Amaudruz et al. (T2K ND280 FGD group),
Nucl.Instrum.Meth. A696, 1 (2012).
[26] N. Abgrall et al. (T2K ND280 TPC group),
Nucl.Instrum.Meth. A637, 25 (2011).
[27] R. B. Patterson et al., Nucl.Instrum.Meth. A608, 206(2009).
[28] K. Abe et al. (T2K Collaboration), Phys.Rev. D88,
032002 (2013).
[29] G. L. Fogli, E. Lisi, A. Marrone, A. Palazzo, and A. M.
Rotunno, Phys.Rev. D84, 053007 (2011).
[30] K. Abe et al. (T2K Collaboration),(2013),
arXiv:1308.0465 [hep-ex].
[31] G.
J.
FeldmanandR.
D.
Cousins,
Phys.Rev. D57, 3873 (1998).