This is the accepted manuscript made available via CHORUS. The article has been
published as:

Indication of Electron Neutrino Appearance from an
Accelerator-Produced Off-Axis Muon Neutrino Beam
K. Abe et al. (T2K Collaboration)
Phys. Rev. Lett. 107, 041801 — Published 18 July 2011
DOI: 10.1103/PhysRevLett.107.041801

Preprint submitted to PRL

Indication of Electron Neutrino Appearance from an
Accelerator-produced Off-axis Muon Neutrino Beam
K. Abe,49 N. Abgrall,16 Y. Ajima,18, ∗ H. Aihara,48 J.B. Albert,13 C. Andreopoulos,47
B. Andrieu,37 S. Aoki,27 O. Araoka,18, ∗ J. Argyriades,16 A. Ariga,3 T. Ariga,3S. Assylbekov,11 D. Autiero,32 A. Badertscher,15 M. Barbi,40 G.J. Barker,56 G. Barr,36
M. Bass,11 F. Bay,3 S. Bentham,29 V. Berardi,22 B.E. Berger,11 I. Bertram,29 M. Besnier,14
J. Beucher,8 D. Beznosko,34 S. Bhadra,59 F.d.M. Blaszczyk,8 A. Blondel,16 C. Bojechko,53
J. Bouchez,8, † S.B. Boyd,56 A. Bravar,16 C. Bronner,14 D.G. Brook-Roberge,5N. Buchanan,11 H. Budd,41 D. Calvet,8 S.L. Cartwright,44 A. Carver,56 R. Castillo,19
M.G. Catanesi,22 A. Cazes,32 A. Cervera,20 C. Chavez,30 S. Choi,43 G. Christodoulou,30
J. Coleman,30 G. Collazuol,24 W. Coleman,31 K. Connolly,57 A. Curioni,15 A. Dabrowska,17
I. Danko,38 R. Das,11 G.S. Davies,29 S. Davis,57 M. Day,41 G. De Rosa,23 J.P.A.M. deAndré,14 P. de Perio,51 A. Delbart,8 C. Densham,47 F. Di Lodovico,39 S. Di Luise,15
P. Dinh Tran,14 J. Dobson,21 U. Dore,25 O. Drapier,14 F. Dufour,16 J. Dumarchez,37
S. Dytman,38 M. Dziewiecki,55 M. Dziomba,57 S. Emery,8 A. Ereditato,3 L. Escudero,20
L.S. Esposito,15 M. Fechner,13, 8 A. Ferrero,16 A.J. Finch,29 E. Frank,3 Y. Fujii,18, ∗Y. Fukuda,33 V. Galymov,59 F. C. Gannaway,39 A. Gaudin,53 A. Gendotti,15 M.A. George,39
S. Giffin,40 C. Giganti,19 K. Gilje,34 T. Golan,58 M. Goldhaber,6, † J.J. Gomez-Cadenas,20
M. Gonin,14 N. Grant,29 A. Grant,46 P. Gumplinger,52 P. Guzowski,21 A. Haesler,16
M.D. Haigh,36 K. Hamano,52 C. Hansen,20, ‡ D. Hansen,38 T. Hara,27 P.F. Harrison,56
B. Hartfiel,31 M. Hartz,59, 51 T. Haruyama,18, ∗ T. Hasegawa,18, ∗ N.C. Hastings,40
S. Hastings,5 A. Hatzikoutelis,29 K. Hayashi,18, ∗ Y. Hayato,49 C. Hearty,5, § R.L. Helmer,52
R. Henderson,52 N. Higashi,18, ∗ J. Hignight,34 E. Hirose,18, ∗ J. Holeczek,45 S. Horikawa,15
A. Hyndman,39 A.K. Ichikawa,28 K. Ieki,28 M. Ieva,19 M. Iida,18, ∗ M. Ikeda,28 J. Ilic,47
J. Imber,34 T. Ishida,18, ∗ C. Ishihara,50 T. Ishii,18, ∗ S.J. Ives,21 M. Iwasaki,48 K. Iyogi,49
A. Izmaylov,26 B. Jamieson,5 R.A. Johnson,10 K.K. Joo,9 G.V. Jover-Manas,19 C.K. Jung,34
H. Kaji,50 T. Kajita,50 H. Kakuno,48 J. Kameda,49 K. Kaneyuki,50, † D. Karlen,53, 52
K. Kasami,18, ∗ I. Kato,52 E. Kearns,4 M. Khabibullin,26 F. Khanam,11 A. Khotjantsev,261

D. Kielczewska,54 T. Kikawa,28 J. Kim,5 J.Y. Kim,9 S.B. Kim,43 N. Kimura,18, ∗ B. Kirby,5
J. Kisiel,45 P. Kitching,1 T. Kobayashi,18, ∗ G. Kogan,21 S. Koike,18, ∗ A. Konaka,52
L.L. Kormos,29 A. Korzenev,16 K. Koseki,18, ∗ Y. Koshio,49 Y. Kouzuma,49 K. Kowalik,2V. Kravtsov,11 I. Kreslo,3 W. Kropp,7 H. Kubo,28 Y. Kudenko,26 N. Kulkarni,31 R. Kurjata,55
T. Kutter,31 J. Lagoda,2 K. Laihem,42 M. Laveder,24 K.P. Lee,50 P.T. Le,34 J.M. Levy,37
C. Licciardi,40 I.T. Lim,9 T. Lindner,5 R.P. Litchfield,56, 28 M. Litos,4 A. Longhin,8G.D. Lopez,34 P.F. Loverre,25 L. Ludovici,25 T. Lux,19 M. Macaire,8 K. Mahn,52
Y. Makida,18, ∗ M. Malek,21 S. Manly,41 A. Marchionni,15 A.D. Marino,10 J. Marteau,32
J.F. Martin,51, § T. Maruyama,18, ∗ T. Maryon,29 J. Marzec,55 P. Masliah,21 E.L. Mathie,40
C. Matsumura,35 K. Matsuoka,28 V. Matveev,26 K. Mavrokoridis,30 E. Mazzucato,8N. McCauley,30 K.S. McFarland,41 C. McGrew,34 T. McLachlan,50 M. Messina,3
W. Metcalf,31 C. Metelko,47 M. Mezzetto,24 P. Mijakowski,2 C.A. Miller,52 A. Minamino,28
O. Mineev,26 S. Mine,7 A.D. Missert,10 G. Mituka,50 M. Miura,49 K. Mizouchi,52
L. Monfregola,20 F. Moreau,14 B. Morgan,56 S. Moriyama,49 A. Muir,46 A. Murakami,28
M. Murdoch,30 S. Murphy,16 J. Myslik,53 T. Nakadaira,18, ∗ M. Nakahata,49 T. Nakai,35
K. Nakajima,35 T. Nakamoto,18, ∗ K. Nakamura,18, ∗ S. Nakayama,49 T. Nakaya,28
D. Naples,38 M.L. Navin,44 B. Nelson,34 T.C. Nicholls,47 K. Nishikawa,18, ∗ H. Nishino,50
J.A. Nowak,31 M. Noy,21 Y. Obayashi,49 T. Ogitsu,18, ∗ H. Ohhata,18, ∗ T. Okamura,18, ∗K. Okumura,50 T. Okusawa,35 S.M. Oser,5 M. Otani,28 R. A. Owen,39 Y. Oyama,18, ∗T. Ozaki,35 M.Y. Pac,12 V. Palladino,23 V. Paolone,38 P. Paul,34 D. Payne,30 G.F. Pearce,47
J.D. Perkin,44 V. Pettinacci,15 F. Pierre,8, † E. Poplawska,39 B. Popov,37, ¶ M. Posiadala,54
J.-M. Poutissou,52 R. Poutissou,52 P. Przewlocki,2 W. Qian,47 J.L. Raaf,4 E. Radicioni,22
P.N. Ratoff,29 T.M. Raufer,47 M. Ravonel,16 M. Raymond,21 F. Retiere,52 A. Robert,37
P.A. Rodrigues,41 E. Rondio,2 J.M. Roney,53 B. Rossi,3 S. Roth,42 A. Rubbia,15
D. Ruterbories,11 S. Sabouri,5 R. Sacco,39 K. Sakashita,18, ∗ F. Sánchez,19 A. Sarrat,8K. Sasaki,18, ∗ K. Scholberg,13 J. Schwehr,11 M. Scott,21 D.I. Scully,56 Y. Seiya,35
T. Sekiguchi,18, ∗ H. Sekiya,49 M. Shibata,18, ∗ Y. Shimizu,50 M. Shiozawa,49 S. Short,21
M. Siyad,47 R.J. Smith,36 M. Smy,7 J.T. Sobczyk,58 H. Sobel,7 M. Sorel,20 A. Stahl,42
P. Stamoulis,20 J. Steinmann,42 B. Still,39 J. Stone,4 C. Strabel,15 L.R. Sulak,4 R. Sulej,2P. Sutcliffe,30 A. Suzuki,27 K. Suzuki,28 S. Suzuki,18, ∗ S.Y. Suzuki,18, ∗ Y. Suzuki,18, ∗2

Y. Suzuki,49 T. Szeglowski,45 M. Szeptycka,2 R. Tacik,40, 52 M. Tada,18, ∗ S. Takahashi,28
A. Takeda,49 Y. Takenaga,49 Y. Takeuchi,27 K. Tanaka,18, ∗ H.A. Tanaka,5, § M. Tanaka,18, ∗M.M. Tanaka,18, ∗ N. Tanimoto,50 K. Tashiro,35 I. Taylor,34 A. Terashima,18, ∗ D. Terhorst,42
R. Terri,39 L.F. Thompson,44 A. Thorley,30 W. Toki,11 T. Tomaru,18, ∗ Y. Totsuka,18, †C. Touramanis,30 T. Tsukamoto,18, ∗ M. Tzanov,31, 10 Y. Uchida,21 K. Ueno,49 A. Vacheret,21
M. Vagins,7 G. Vasseur,8 T. Wachala,17 J.J. Walding,21 A.V. Waldron,36 C.W. Walter,13
P.J. Wanderer,6 J. Wang,48 M.A. Ward,44 G.P. Ward,44 D. Wark,47, 21 M.O. Wascko,21
A. Weber,36, 47 R. Wendell,13 N. West,36 L.H. Whitehead,56 G. Wikström,16 R.J. Wilkes,57
M.J. Wilking,52 J.R. Wilson,39 R.J. Wilson,11 T. Wongjirad,13 S. Yamada,49 Y. Yamada,18, ∗A. Yamamoto,18, ∗ K. Yamamoto,35 Y. Yamanoi,18, ∗ H. Yamaoka,18, ∗ C. Yanagisawa,34, ∗∗T. Yano,27 S. Yen,52 N. Yershov,26 M. Yokoyama,48 A. Zalewska,17 J. Zalipska,5
L. Zambelli,37 K. Zaremba,55 M. Ziembicki,55 E.D. Zimmerman,10 M. Zito,8 and J. Żmuda58(The T2K Collaboration)
1

University of Alberta, Centre for Particle Physics,

Department of Physics, Edmonton, Alberta, Canada
2
3

The Andrzej Soltan Institute for Nuclear Studies, Warsaw, Poland

University of Bern, Albert Einstein Center for Fundamental Physics,
Laboratory for High Energy Physics (LHEP), Bern, Switzerland

4

Boston University, Department of Physics, Boston, Massachusetts, U.S.A.
5

University of British Columbia, Department of Physics

and Astronomy, Vancouver, British Columbia, Canada
6

Brookhaven National Laboratory, Physics Department, Upton, New York, U.S.A.
7

University of California, Irvine, Department of

Physics and Astronomy, Irvine, California, U.S.A.
8

IRFU, CEA Saclay, Gif-sur-Yvette, France

9

Chonnam National University, Institute for

Universe & Elementary Particles, Gwangju, Korea
10

University of Colorado at Boulder, Department of Physics, Boulder, Colorado, U.S.A.
11

Colorado State University, Department of Physics, Fort Collins, Colorado, U.S.A.
12

Dongshin University, Department of Physics, Naju, Korea
3

13
14

Duke University, Department of Physics, Durham, North Carolina, U.S.A.

Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, Palaiseau, France
15
16

University of Geneva, Section de Physique, DPNC, Geneva, Switzerland

17
18

ETH Zurich, Institute for Particle Physics, Zurich, Switzerland

H. Niewodniczanski Institute of Nuclear Physics PAN, Cracow, Poland

High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan
19

Institut de Fisica d’Altes Energies (IFAE), Bellaterra (Barcelona), Spain
20

21

IFIC (CSIC & University of Valencia), Valencia, Spain

Imperial College London, Department of Physics, London, United Kingdom
22

INFN Sezione di Bari and Università e Politecnico diBari, Dipartimento Interuniversitario di Fisica, Bari, Italy
23
24

INFN Sezione di Napoli and Università di Napoli, Dipartimento di Fisica, Napoli, Italy

INFN Sezione di Padova and Università di Padova, Dipartimento di Fisica, Padova, Italy
25
26

INFN Sezione di Roma and Università di Roma ”La Sapienza”, Roma, Italy

Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
27
28
29
30

Kobe University, Kobe, Japan

Kyoto University, Department of Physics, Kyoto, Japan

Lancaster University, Physics Department, Lancaster, United Kingdom

University of Liverpool, Department of Physics, Liverpool, United Kingdom
31

Louisiana State University, Department of Physics

and Astronomy, Baton Rouge, Louisiana, U.S.A.
32

Université de Lyon, Université Claude Bernard

Lyon 1, IPN Lyon (IN2P3), Villeurbanne, France
33

Miyagi University of Education, Department of Physics, Sendai, Japan
34

State University of New York at Stony Brook, Departmentof Physics and Astronomy, Stony Brook, New York, U.S.A.
35
36

Osaka City University, Department of Physics, Osaka, Japan

Oxford University, Department of Physics, Oxford, United Kingdom
37

UPMC, Université Paris Diderot, CNRS/IN2P3, Laboratoire dePhysique Nucléaire et de Hautes Energies (LPNHE), Paris, France
38

University of Pittsburgh, Department of Physics

and Astronomy, Pittsburgh, Pennsylvania, U.S.A.
4

39

Queen Mary University of London, School of Physics, London, United Kingdom
40

University of Regina, Physics Department, Regina, Saskatchewan, Canada
41

University of Rochester, Department of Physics

and Astronomy, Rochester, New York, U.S.A.
42

43

44

RWTH Aachen University, III. Physikalisches Institut, Aachen, Germany

Seoul National University, Department of Physics and Astronomy, Seoul, Korea

University of Sheffield, Department of Physics and Astronomy, Sheffield, United Kingdom
45

University of Silesia, Institute of Physics, Katowice, Poland

46

47

STFC, Daresbury Laboratory, Warrington, United Kingdom

STFC, Rutherford Appleton Laboratory, Harwell Oxford, United Kingdom
48

University of Tokyo, Department of Physics, Tokyo, Japan
49

University of Tokyo, Institute for Cosmic Ray

Research, Kamioka Observatory, Kamioka, Japan
50

University of Tokyo, Institute for Cosmic Ray Research,

Research Center for Cosmic Neutrinos, Kashiwa, Japan
51

University of Toronto, Department of Physics, Toronto, Ontario, Canada
52

53

TRIUMF, Vancouver, British Columbia, Canada

University of Victoria, Department of Physics andAstronomy, Victoria, British Columbia, Canada

54

55

Warsaw University of Technology, Institute of Radioelectronics, Warsaw, Poland
56

57

University of Warwick, Department of Physics, Coventry, United Kingdom

University of Washington, Department of Physics, Seattle, Washington, U.S.A.
58

59

University of Warsaw, Faculty of Physics, Warsaw, Poland

Wroclaw University, Faculty of Physics and Astronomy, Wroclaw, Poland

York University, Department of Physics and Astronomy, Toronto, Ontario, Canada
5

Abstract
The T2K experiment observes indications of νµ → νe appearance in data accumulated with
1.43 × 1020 protons on target. Six events pass all selection criteria at the far detector. In a threeflavor neutrino oscillation scenario with |∆m223 | = 2.4 × 10−3 eV2 , sin2 2θ23 = 1 and sin2 2θ13 = 0,
the expected number of such events is 1.5±0.3(syst.). Under this hypothesis, the probability toobserve six or more candidate events is 7×10−3 , equivalent to 2.5σ significance. At 90% C.L.,
the data are consistent with 0.03(0.04)< sin2 2θ13 < 0.28(0.34) for δCP = 0 and normal (inverted)hierarchy.
PACS numbers: 14.60.Pq,13.15.+g,25.30.Pt,95.55.Vj

6

We report results of a search for νe appearance in the T2K experiment [1]. In a threeneutrino mixing scenario, flavor oscillations are described by the PMNS matrix [2, 3], usually
parametrized by the three angles θ12 , θ23 , θ13 , and the CP-violating phase δCP . Previous
experiments have observed neutrino oscillations driven by θ12 and θ23 in the solar (∆m212 )and atmospheric (∆m213 '∆m223 ) sectors [4–9]. In the atmospheric sector, data are consistent
with |∆m223 | ' 2.4 × 10−3 eV2 , a normal ∆m223 > 0 or inverted ∆m223 < 0 mass hierarchy,and sin2 2θ23 close to, or equal to unity. Searches for oscillations driven by θ13 have been
inconclusive and upper limits have been derived [10–13], with the most stringent beingsin2 2θ13 <0.15 (90%C.L.), set by CHOOZ [14] and MINOS [15].
T2K uses a conventional neutrino beam produced at J-PARC and directed 2.5◦ off-axis toSuper-Kamiokande (SK) at a distance L = 295 km. This configuration produces a narrowband νµ beam [16], tuned at the first oscillation maximum Eν =|∆m223 | L/(2π) ' 0.6 GeV,
reducing backgrounds from higher energy neutrino interactions.
Details of the T2K experimental setup are described elsewhere [17]. Here we briefly review
the components relevant for the νe search. The J-PARC Main Ring (MR) accelerator [18]provides 30 GeV protons with a cycle of 0.3 Hz. Eight bunches are single-turn extracted in
5 µs and transported through an extraction line arc defined by superconducting combinedfunction magnets to the production target. The primary beamline is equipped with 21
electrostatic beam position monitors (ESM), 19 segmented secondary emission monitors(SSEM), one optical transition radiation monitor (OTR) and five current transformers. The
secondary beamline, filled with He at atmospheric pressure, is composed of the target,
focusing horns and decay tunnel. The graphite target is 2.6 cm in diameter and 90 cm(1.9λint ) long. Charged particles exiting the target are sign selected and focused into the
96 m long decay tunnel by three magnetic horns pulsed at 250 kA. Neutrinos are primarily
produced in the decays of charged pions and kaons. A beam dump is located at the end of
the tunnel and is followed by muon monitors.
The Near Detector complex [17] located 280 m downstream from the target hosts two
detectors. The on-axis Interactive Neutrino GRID (INGRID) accumulates neutrino interactions with high statistics to monitor the beam intensity, direction and profile. It consistsof 14 identical 7-ton iron-absorber/scintillator-tracker sandwich modules arranged in 10 m
by 10 m crossed horizontal and vertical arrays centered on the beam. The off-axis detector
reconstructs exclusive final states to study neutrino interactions and beam properties corre7

sponding to those expected at the far detector. Embedded in the refurbished UA1 magnet(0.2 T), it consists of three large volume time projection chambers (TPCs) [19] interleavedwith two fine-grained tracking detectors (FGDs, each 1 ton), a π 0 -optimized detector and asurrounding electromagnetic calorimeter. The magnet yoke is instrumented as a side muon
range detector.
The SK water Cherenkov far detector [20] has a fiducial volume (FV) of 22.5 kton within
its cylindrical inner detector (ID). Enclosing the ID all around is the 2 m-wide outer detector(OD). The front-end readout electronics allow for a zero-deadtime software trigger. Spilltiming information, synchronized by the Global Positioning System (GPS) with < 150 ns
precision, is transferred online to SK and triggers the recording of photomultiplier hits within±500 µs of the expected arrival time of the neutrinos.
The results presented in this Letter are based on the first two physics runs: Run 1 (Jan–Jun 2010) and Run 2 (Nov 2010–Mar 2011). During this time period, the MR proton beam
power was continually increased and reached 145 kW with 9 × 1013 protons per pulse. The
targeting efficiency was monitored by the ESM, SSEM and OTR and found to be stableat over 99%. The muon monitors provided additional spill-by-spill steering information. A
total of 2,474,419 spills were retained for analysis after beam and far detector quality cuts,
yielding 1.43 × 1020 protons on target (p.o.t.).
We present the study of events in the far detector with only a single electron-like (elike) ring. The analysis produces a sample enhanced in νe charged-current quasi-elastic
interactions (CCQE) arising from νµ → νe oscillations. The main backgrounds are intrinsic
νe contamination in the beam and neutral current (NC) interactions with a misidentified
π 0 . The selection criteria for this analysis were fixed from Monte Carlo (MC) studies before
the data were collected, optimized for the initial running conditions. The observed numberof events is compared to expectations based on neutrino flux and cross-section predictions
for signal and all sources of backgrounds, which are corrected using an inclusive νµ chargedcurrent (CC) measurement in the off-axis near detector.
We compute the neutrino beam fluxes (Fig. 1) starting from models and tuning them toexperimental data. Pion production in (p, θ) bins is based on the NA61 measurements [21],typically with 5–10% uncertainties. Pions produced outside the experimentally measured
phase space, as well as kaons, are modeled using FLUKA [22, 23]. These pions are assigned
systematic uncertainties on their production of 50%, while kaon production uncertainties,
8Flux /(cm 2 ⋅ 1021 POT ⋅ 100 MeV)

νµ
νµ
νe
νe

6

10

105
104
103

0

1

2
3
4
Neutrino Energy (GeV)

5

FIG. 1. Predicted neutrino fluxes at the far detector, in absence of oscillations. The shaded boxes
indicate the total systematic uncertainties for each energy bin.

estimated from a comparison with data from Eichten et al. [24], range from 15% to 100%
depending on the bin. GEANT3 [25], with GCALOR [26] for hadronic interactions, handles
particle propagation through the magnetic horns, target hall, decay volume and beam dump.
Additional errors to the neutrino fluxes are included for the proton beam uncertainties,
secondary beamline component alignment uncertainties, and the beam direction uncertainty.
The neutrino beam profile and its absolute rate (1.5 events/1014 p.o.t.) as measured by
INGRID were stable and consistent with expectations. The beam profile center (Fig. 2)
indicates that beam steering was better than ±1 mrad. The correlated systematic error is±0.33(0.37) mrad for the horizontal(vertical) direction. The error on the SK position relative
to the beamline elements was obtained from a dedicated GPS survey and is negligible. As
shown in Fig. 1, the estimated uncertainties of the intrinsic νµ and νe fluxes below 1 GeV
are around 14%. Above 1 GeV, the intrinsic νe flux error is dominated by the uncertainty
on the kaon production rate with resulting errors of 20–50%.
The NEUT MC event generator [28], which has been tuned with recent neutrino interaction data in an energy region compatible with T2K [29–31], is used to simulate neutrino
interactions in the near and far detectors. The GENIE [32] generator provides a separate
9Profile center [cm]

S(U)

30

X centerY center+- 1mrad

20
10
0
-10
-20
-30

N(D) Jan,FM
eb 2 ar
010

Apr

May

Jun

Dec

Feb

201

1

Mar

FIG. 2. Beam centering stability in horizontal (x, South–North) and vertical (y, Down–Up) directions as a function of time, as measured by INGRID. Errors shown are only statistical. The dashedlines correspond to a change of beam direction by ±1 mrad.

TABLE I. Summary of systematic uncertainties for the relative rate of different charged-current(CC) and neutral-current (NC) reactions to the rate for CCQE.
Process

Systematic error

CCQE

energy-dependent (7% at 500 MeV)

CC 1π

30% (Eν < 2 GeV) – 20% (Eν > 2 GeV)

CC coherent π ±100% (upper limit from [27])CC other

30% (Eν < 2 GeV) – 25% (Eν > 2 GeV)

NC 1π 030% (Eν < 1 GeV) – 20% (Eν > 1 GeV)

NC coherent π

30%

NC other π

30%

FSI

energy-dependent (10% at 500 MeV)

cross-check of the assumed cross-sections and uncertainties, and yields consistent results. A
list of reactions and their uncertainties relative to the CCQE total cross-section is shown in
Table I. An energy-dependent error on CCQE is assigned to account for the uncertainty inthe low energy cross-section, especially for the different target materials between the nearand far detectors. Uncertainties in intranuclear final state interactions (FSI), implemented
with a microscopic cascade model [33], introduce an additional error in the rates (see e.g.
10[34]).
An inclusive νµ CC measurement in the off-axis near detector is used to constrain the
expected event rate at the far detector. From a data sample collected in Run 1 and corresponding to 2.88 × 1019 p.o.t. after detector quality cuts, neutrino interactions are selectedin the FGDs with tracks entering the downstream TPC. The most energetic negative trackin the TPC is chosen and we require its ionization loss to be compatible with a muon. To
reduce background from interactions outside the FGDs, there must be no track in the upstream TPC. The analysis selects 1529 data events (38% νµ CC efficiency for 90% purity,
estimated from MC). The momentum distribution of the selected muons (Fig. 3) shows good
agreement between data and MC. The measured data/MC ratio isµ,Dataµ,M C
RN/RN= 1.036 ± 0.028(stat.)+0.044
DD−0.037 (det.syst.) ± 0.038(phys.syst.),

(1)

µ,M C
µ,Dataare the p.o.t. normalized rates of νµ CC interactions in data andand RN
where RN
D
D

MC. The detector systematic errors mainly come from tracking and particle identification
efficiencies, and physics uncertainties are related to the interaction modeling. Uncertainties
that effectively cancel between near and far detectors were omitted.
At the far detector, we extract a fully-contained fiducial volume (FCFV) sample byrequiring no event activity in either the OD or in the 100 µs before the event trigger time, atleast 30 MeV electron-equivalent energy deposited in the ID (defined as visible energy Evis ),and the reconstructed vertex in the fiducial region. The data have 88 such FCFV events
that are within the timing range from −2 to 10 µs around the beam trigger time. The
accidental contamination from non-beam related events is determined from the sidebands
to be 0.003 events. A Kolmogorov-Smirnov (KS) test of the observed number of FCFV
events as a function of accumulated p.o.t. is compatible with the normalized event rate
being constant (p-value=0.32). The analysis relies on the well-established reconstruction
techniques developed for other data samples [4]. Forty-one events are reconstructed witha single ring, and eight of those are e-like. Six of these events have Evis > 100 MeV
and no delayed-electron signal. To suppress misidentified π 0 mesons, the reconstruction of
two rings is forced by comparison of the observed and expected light patterns calculated
under the assumption of two showers [35], and a cut on the two-ring invariant mass Minv <105 MeV/c2 is imposed. No events are rejected (Fig. 4).

Finally, the neutrino energyEνrec is computed using the reconstructed momentum and direction of the ring, by assuming
11

entries/(100 MeV/c)

200
180

iµ CC QE
iµ CC non QE
NCiµ CC
Outside FGD

160
140
120
100
80
60
40
20
0
0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Muon Momentum (MeV/c)

FIG. 3. Measured muon momentum of νµ CC candidates reconstructed in the FGD target. The
data are shown using points with error bars (statistical only) and the MC predictions are inhistograms shaded according to their type.

quasi-elastic kinematics and neglecting Fermi motion. No events are rejected by requiring
Eνrec < 1250 MeV, aimed at suppressing events from the intrinsic νe component arising
primarily from kaon decays (Fig. 5). The data and MC reductions after each selection
criterion are shown in Table II. The νe appearance signal efficiency is estimated from MC
to be 66% while rejection for νµ + ν̄µ CC, intrinsic νe CC, and NC are > 99%, 77%, and99%, respectively. Of the surviving background NC interactions constitute 46%, of which
74% are due to π 0 mesons and 6% originate from single gamma production.
Examination of the six data events shows properties consistent with νe CC interactions.
The distribution of the cosine of the opening angle between the ring and the incoming beam
direction is consistent with CCQE events. The event vertices in cylindrical coordinates(R,φ,z) show that these events are clustered at large R, near the edge of the FV in the
upstream beam direction. A KS test on the R2 distribution of our final events yields a
p-value of 0.03. If this was related to contamination from penetrating particles produced
in upstream neutrino interactions, then the ID region outside the FV should show evidence
for such events, however this is not observed. In addition, an analysis of the neutrino
interactions occurring in the OD volume is consistent with expectations.
12Number of events /(15 MeV/c2)

5

Data
Osc. νe CC
νμ+νμ CC
νe CC
NC

4
3
2
1
0

0

100

200

2

Invariant mass (MeV/c )300

FIG. 4. Distribution of invariant mass Minv when each event is forced to be reconstructed into tworings. The data are shown using points with error bars (statistical only) and the MC predictions
are in shaded histograms, corresponding to oscillated νe CC signal and various background sources
for sin2 2θ13 = 0.1. The last bin shows overflow entries. The vertical line shows the applied cut at
105 MeV/c2 .
exp
To compute the expected number of events at the far detector NSK, we use the neardetector νµ CC interaction rate measurement as normalization, and the ratio of expected
events in the near and far detectors, where common systematic errors cancel. Using Eq. 1,
this can be expressed as:




exp
µ,Data
µ,M C
MC
NSK
= RN
/RN
· NSK
,
D
D

(2)

MC
where NSK
is the MC number of events expected in the far detector. Due to the correlationof systematic errors in the near and far detector samples, Eq. 2 reduces the uncertainty on the
expected number of events. Event rates are computed incorporating three-flavor oscillation
13

Number of events /(250 MeV)

Data
Osc. νe CC
νμ+νμ CC
νe CC
NC

3

2

1

0

0

1000

2000

3000

Reconstructed ν energy (MeV)

FIG. 5. Same as Fig. 4 for the reconstructed neutrino energy spectrum of the events which passall νe appearance signal selection criteria with the exception of the energy cut. The vertical line
shows the applied cut at 1250 MeV.

probabilities and matter effects [36] with ∆m212 = 7.6 × 10−5 eV2 , ∆m223 = +2.4 × 10−3 eV2 ,sin2 2θ12 = 0.8704, sin2 2θ23 = 1.0, an average Earth density ρ=3.2 g/cm3 and δCP = 0 unlessotherwise noted. The expectations are 0.03(0.03) νµ + ν̄µ CC, 0.8(0.7) intrinsic νe CC, and
0.1(4.1) νµ → νe oscillation events for sin2 2θ13 =0(0.1), and 0.6 NC events. As shown in Taexp
ble III, the total systematic uncertainty on NSK
depends on θ13 . Neutrino flux uncertainties

contribute 14.9%(15.4%) to the far(near) event rates, but their ratio has an 8.5% error due tocancellations. The near detector νµ CC selection efficiency uncertainty yields +5.6
−5.2 % and thestatistical uncertainty gives 2.7%. The errors from cross-section modeling are dominatedby FSI uncertainties and by the knowledge of the σ(νe )/σ(νµ ) ratio, estimated to ±6%.
The systematic uncertainties due to event selection in SK were studied with cosmic-ray
muons, electrons from muon decays, and atmospheric neutrino events. Their contribution
14

TABLE II. Event reduction for the νe appearance search at the far detector. After each selection
criterion is applied, the numbers of observed (Data) and MC expected events of νµ CC, intrinsic
νe CC, NC, and the νe CC signal, are given. All MC CC samples include three-flavor oscillations
for sin2 2θ13 =0.1 and δCP = 0.
Data νµ CC νe CC NC νµ →νe CC(0) interaction in FV

n/a

67.2

3.1 71.0

6.2(1) fully-contained FV

88

52.4

2.9 18.3

6.0(2) single ring

41

30.8

1.8

5.7

5.2

(3) e-like

8

1.0

1.8

3.7

5.2

(4) Evis > 100 MeV

7

0.7

1.8

3.2

5.1

(5) no delayed electron

6

0.1

1.5

2.8

4.6

(6) non-π 0 -like

6

0.04

1.1

0.8

4.2

(7) Eνrec < 1250 MeV

6

0.03

0.7

0.6

4.1

exp
exp
for e.g. sin2 2θ13 = 0.1 is as follows: 1.4% from the fiducial volume definition,/NSK
to δNSK

0.6% from the energy scale and 0.2% from the delayed electron signal tagging efficiency. The
π 0 rejection efficiency, studied with a NC π 0 topological control sample combining one data
electron and one simulated gamma event, contributes 0.9%. The uncertainty on the acceptance of one-ring e-like events was studied with an atmospheric neutrino sample, adding acontribution of 5% from ring counting and 4.9% from particle identification uncertainties.
The performance of muon rejection by the ring particle identification algorithm was investigated using cosmic-ray muons and atmospheric neutrino events, giving 0.3%. The effectfrom uncertainties in the Minv cut is 6.0%. Combining the above uncertainties, the total far
exp
exp
detector systematic error contribution to δNSK
/NSK
is 14.7%(9.4%) for sin2 2θ13 = 0(0.1).

Our oscillation result is based entirely on comparing the number of νe candidate eventswith predictions, varying sin2 2θ13 for each δCP value. Including systematic uncertainties,
the expectation is 1.5±0.3(5.5±1.0) events for sin2 2θ13 = 0(0.1). At each oscillation parameter point, a probability distribution for the expected number of events is constructed,
incorporating systematic errors [37], which is used to make the confidence interval (Fig. 6),
following the unified ordering prescription of Feldman and Cousins [38].
In conclusion, the observation of six single ring e-like events exceeds the expectation of
15

TABLE III. Contributions from various sources and the total relative uncertainty for sin2 2θ13 =0
and 0.1, and δCP = 0.
sin2 2θ13 = 0

sin2 2θ13 = 0.1(1) neutrino flux

± 8.5%± 8.5%(2) near detector+5.6−5.2 %+5.6−5.2 %(3) near det. statistics± 2.7%± 2.7%(4) cross section± 14.0%± 10.5%(5) far detector± 14.7%± 9.4%+22.8−22.7 %

+17.6
−17.5 %

Sourceexp
exp
Total δNSK
/NSK

a three-flavor neutrino oscillation scenario with sin2 2θ13 = 0. Under this hypothesis, theprobability to observe six or more candidate events is 7×10−3 . Thus, we conclude thatour data indicate νe appearance from a νµ neutrino beam. This result converted into a
confidence interval yields 0.03(0.04) < sin2 2θ13 < 0.28(0.34) at 90% C.L. for sin2 2θ23 = 1.0,|∆m223 | = 2.4×10−3 eV2 , δCP = 0 and for normal (inverted) neutrino mass hierarchy. Under
the same assumptions, the best fit points are 0.11(0.14), respectively. For non-maximal
sin2 2θ23 , the confidence intervals remain unchanged to first order by replacing sin2 2θ13 by
2 sin2 θ23 sin2 2θ13 . More data are required to firmly establish νe appearance and to better
determine the angle θ13 .
We thank the J-PARC accelerator team for the superb accelerator performance and
CERN NA61 colleagues for providing essential particle production data and for their excellent collaboration. 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 Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MEST and NRF, South Korea; MICINN and CPAN, Spain;
SNSF and SER, Switzerland; STFC, U.K.; and DOE, U.S.A. We also thank CERN for their
donation of the UA1/NOMAD magnet and DESY for the HERA-B magnet mover system.
In addition, participation of individual researchers and institutions in T2K has been further
supported by funds from: ERC (FP7), EU; JSPS, Japan; Royal Society, UK; DOE-OJI and
DOE-Early Career program, and the A. P. Sloan Foundation, U.S.A.
16

π

Δ m223 > 0

δCP

π/2
0

Best fit to T2K data
68% CL
90% CL-π/2-ππ

Δ m223 < 0

δCP

π/2
0

T2K
1.43 ×1020 p.o.t.

-π/2
-π

0

0.1

0.2

0.3

sin22θ13

0.4

0.5

0.6

FIG. 6. The 68% and 90% C.L. regions for sin2 2θ13 for each value of δCP , consistent with theobserved number of events in the three-flavor oscillation case for normal (top) and inverted (bottom)
mass hierarchy. The other oscillation parameters are fixed (see text). The best fit values are shownwith solid lines.

∗

also at J-PARC Center

17†deceased‡

now at CERN§

also at Institute of Particle Physics, Canada¶

also at JINR, Dubna, Russia∗∗

also at BMCC/CUNY, New York, New York, U.S.A.

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20