The T2K Experiment

The T2K Experiment

K. Abe^aw, N. Abgrall^p, H. Aihara^av, Y. Ajima^r, J.B. Albert^m, D. Allan^au, P.-A. Amaudruz^az, C. Andreopoulos^au, B. Andrieu^ak, M.D. Anerella^f, C. Angelsen^au, S. Aoki^aa, O. Araoka^r, J. Argyriades^p, A. Ariga^c, T. Ariga^c, S. Assylbekov^k, J.P.A.M. de Andr´eⁿ, D. Autiero^af, A. Badertscher^o, O. Ballester^s, M. Barbi^an, G.J. Barker^bd, P. Baron^h, G. Barr^aj, L. Bartoszek^j, M. Batkiewicz^q, F. Bay^c,

S. Bentham^ac, V. Berardi^v, B.E. Berger^k, H. Berns^be, I. Bertram^ac, M. Besnierⁿ, J. Beucher^h, D. Beznosko^ah, S. Bhadra^bg, P. Birney^ba,az, D. Bishop^az, E. Blackmore^az, F.d.M. Blaszczyk^h, J. Blocki^q, A. Blondel^p, A. Bodek^ao, C. Bojechko^ba, J. Bouchez^h,†,

T. Boussuge^h, S.B. Boyd^bd, M. Boyer^h, N. Braam^ba, R. Bradford^ao, A. Bravar^p, K. Briggs^bd, J.D. Brinson^ae, C. Bronnerⁿ, D.G. Brook-Roberge^e, M. Bryant^e, N. Buchanan^k, H. Budd^ao, M. Cadabeschi^ay, R.G. Calland^ad, D. Calvet^h, J. Caravaca Rodr´ıguez^s,

J. Carroll^ad, S.L. Cartwright^ar, A. Carver^bd, R. Castillo^s, M.G. Catanesi^v, C. Cavata^h, A. Cazes^af, A. Cervera^t, J.P. Charrier^h, C. Chavez^ad, S. Choi^aq, S. Cholletⁿ, G. Christodoulou^ad, P. Colas^h, J. Coleman^ad, W. Coleman^ae, G. Collazuol^x, K. Connolly^be, P. Cooke^ad, A. Curioni^o, A. Dabrowska^q, I. Danko^al, R. Das^k, G.S. Davies^ac, S. Davis^be, M. Day^ao, X. De La Broise^h, P. de Perio^ay, G. De Rosa^w, T. Dealtry^aj,au, A. Debraineⁿ, E. Delagnes^h, A. Delbart^h, C. Densham^au, F. Di Lodovico^am, S. Di Luise^o, P. Dinh Tranⁿ,

J. Dobson^u, J. Doornbos^az, U. Dore^y, O. Drapierⁿ, F. Druillole^h, F. Dufour^p, J. Dumarchez^ak, T. Durkin^au, S. Dytman^al, M. Dziewiecki^bc, M. Dziomba^be, B. Ellison^ae, S. Emery^h, A. Ereditato^c, J.E. Escallier^f, L. Escudero^t, L.S. Esposito^o, W. Faszer^az,

M. Fechner^m, A. Ferrero^p, A. Finch^ac, C. Fisher^az, M. Fitton^au, R. Flight^ao, D. Forbush^be, E. Frank^c, K. Fransham^ba, Y. Fujii^r, Y. Fukuda^ag, M. Gallop^az, V. Galymov^bg, G.L. Ganetis^f, F.C. Gannaway^am, A. Gaudin^ba, J. Gaweda^s, A. Gendotti^o, M. George^am,

S. Giffin^an, C. Giganti^s,h, K. Gilje^ah, I. Giomataris^h, J. Giraud^h, A.K. Ghosh^f, T. Golan^bf, M. Goldhaber^f,†, J.J. Gomez-Cadenas^t, S. Gomi^ab, M. Goninⁿ, M. Goyette^az, A. Grant^at, N. Grant^au, F. Gra˜nena^s, S. Greenwood^u, P. Gumplinger^az, P. Guzowski^u, M.D. Haigh^aj, K. Hamano^az, C. Hansen^t, T. Hara^aa, P.F. Harrison^bd, B. Hartfiel^ae, M. Hartz^bg,ay, T. Haruyama^r, R. Hasanen^ba,

T. Hasegawa^r, N.C. Hastings^an, S. Hastings^e, A. Hatzikoutelis^ac, K. Hayashi^r, Y. Hayato^aw, T.D.J. Haycock^ar, C. Hearty^e,2, R.L. Helmer^az, R. Henderson^az, S. Herlant^h, N. Higashi^r, J. Hignight^ah, K. Hiraide^ab, E. Hirose^r, J. Holeczek^as, N. Honkanen^ba,

S. Horikawa^o, A. Hyndman^am, A.K. Ichikawa^ab, K. Ieki^ab, M. Ieva^s, M. Iida^r, M. Ikeda^ab, J. Ilic^au, J. Imber^ah, T. Ishida^r, C. Ishihara^ax, T. Ishii^r, S.J. Ives^u, M. Iwasaki^av, K. Iyogi^aw, A. Izmaylov^z, B. Jamieson^e, R.A. Johnson^j, K.K. Jooⁱ, G. Jover-Manas^s,

C.K. Jung^ah, H. Kaji^ax, T. Kajita^ax, H. Kakuno^av, J. Kameda^aw, K. Kaneyuki^ax,†, D. Karlen^ba,az, K. Kasami^r, V. Kasey^u, I. Kato^az, H. Kawamuko^ab, E. Kearns^d, L. Kellet^ad, M. Khabibullin^z, M. Khaleeq^u, N. Khan^az, A. Khotjantsev^z, D. Kielczewska^bb, T. Kikawa^ab, J.Y. Kimⁱ, S.-B. Kim^aq, N. Kimura^r, B. Kirby^e, J. Kisiel^as, P. Kitching^a, T. Kobayashi^r, G. Kogan^u, S. Koike^r, T. Komorowski^ac, A. Konaka^az, L.L. Kormos^ac, A. Korzenev^p, K. Koseki^r, Y. Koshio^aw, Y. Kouzuma^aw, K. Kowalik^b, V. Kravtsov^k,

I. Kreslo^c, W. Kropp^g, H. Kubo^ab, J. Kubota^ab, Y. Kudenko^z, N. Kulkarni^ae, L. Kurchaninov^az, Y. Kurimoto^ab, R. Kurjata^bc, Y. Kurosawa^ab, T. Kutter^ae, J. Lagoda^b, K. Laihem^ap, R. Langstaff^ba,az, M. Laveder^x, T.B. Lawson^ar, P.T. Le^ah, A. Le Coguie^h, M. Le Ross^az, K.P. Lee^ax, M. Lenckowski^ba,az, C. Licciardi^an, I.T. Limⁱ, T. Lindner^e, R.P. Litchfield^bd,ab, A. Longhin^h, G.D. Lopez^ah, P. Lu^e, L. Ludovici^y, T. Lux^s, M. Macaire^h, L. Magaletti^v, K. Mahn^az, Y. Makida^r, C.J. Malafis^ah, M. Malek^u, S. Manly^ao, A. Marchionni^o,

C. Mark^az, A.D. Marino^j,ay, A.J. Marone^f, J. Marteau^af, J.F. Martin^ay,2, T. Maruyama^r, T. Maryon^ac, J. Marzec^bc, P. Masliah^u, E.L. Mathie^an, C. Matsumura^ai, K. Matsuoka^ab, V. Matveev^z, K. Mavrokoridis^ad, E. Mazzucato^h, N. McCauley^ad, K.S. McFarland^ao, C. McGrew^ah, T. McLachlan^ax, I. Mercer^ac, M. Messina^c, W. Metcalf^ae, C. Metelko^au, M. Mezzetto^x, P. Mijakowski^b, C.A. Miller^az,

A. Minamino^ab, O. Mineev^z, S. Mine^g, R.E. Minvielle^ae, G. Mituka^ax, M. Miura^aw, K. Mizouchi^az, J.-P. Mols^h, L. Monfregola^t, E. Monmarthe^h, F. Moreauⁿ, B. Morgan^bd, S. Moriyama^aw, D. Morris^az, A. Muir^at, A. Murakami^ab, J.F. Muratore^f, M. Murdoch^ad,

S. Murphy^p, J. Myslik^ba, G. Nagashima^ah, T. Nakadaira^r, M. Nakahata^aw, T. Nakamoto^r, K. Nakamura^r, S. Nakayama^aw, T. Nakaya^ab, D. Naples^al, B. Nelson^ah, T.C. Nicholls^au, K. Nishikawa^r, H. Nishino^ax, K. Nitta^ab, F. Nizery^h, J.A. Nowak^ae, M. Noy^u,

Y. Obayashi^aw, T. Ogitsu^r, H. Ohhata^r, T. Okamura^r, K. Okumura^ax, T. Okusawa^ai, C. Ohlmann^az, K. Olchanski^az, R. Openshaw^az, S.M. Oser^e, M. Otani^ab, R.A. Owen^am, Y. Oyama^r, T. Ozaki^ai, M.Y. Pac^l, V. Palladino^w, V. Paolone^al, P. Paul^ah, D. Payne^ad, G.F. Pearce^au, C. Pearson^az, J.D. Perkin^ar, M. Pfleger^ba, F. Pierre^h,†, D. Pierrepont^h, P. Plonski^bc, P. Poffenberger^ba, E. Poplawska^am,

B. Popov^ak,1, M. Posiadala^bb, J.-M. Poutissou^az, R. Poutissou^az, R. Preece^au, P. Przewlocki^b, W. Qian^au, J.L. Raaf^d, E. Radicioni^v, K. Ramos^ah, P. Ratoff^ac, T.M. Raufer^au, M. Ravonel^p, M. Raymond^u, F. Retiere^az, D. Richards^bd, J.-L. Ritou^h, A. Robert^ak, P.A. Rodrigues^ao, E. Rondio^b, M. Roney^ba, M. Rooney^au, D. Ross^az, B. Rossi^c, S. Roth^ap, A. Rubbia^o, D. Ruterbories^k, R. Sacco^am,

S. Sadler^ar, K. Sakashita^r, F. Sanchez^s, A. Sarrat^h, K. Sasaki^r, P. Schaack^u, J. Schmidt^ah, K. Scholberg^m, J. Schwehr^k, M. Scott^u, D.I. Scully^bd, Y. Seiya^ai, T. Sekiguchi^r, H. Sekiya^aw, G. Sheffer^az, M. Shibata^r, Y. Shimizu^ax, M. Shiozawa^aw, S. Short^u, M. Siyad^au,

D. Smith^ae, R.J. Smith^aj, M. Smy^g, J. Sobczyk^bf, H. Sobel^g, S. Sooriyakumaran^az, M. Sorel^t, J. Spitz^j, A. Stahl^ap, P. Stamoulis^t, O. Star^az, J. Statter^ac, L. Stawnyczy^bg, J. Steinmann^ap, J. Steffens^ah, B. Still^am, M. Stodulski^q, J. Stone^d, C. Strabel^o, T. Strauss^o,

R. Sulej^b,bc, P. Sutcliffe^ad, A. Suzuki^aa, K. Suzuki^ab, S. Suzuki^r, S.Y. Suzuki^r, Y. Suzuki^r, Y. Suzuki^aw, J. Swierblewski^q, T. Szeglowski^as, M. Szeptycka^b, R. Tacik^an, M. Tada^r, A.S. Tadepalli^ah, M. Taguchi^ab, S. Takahashi^ab, A. Takeda^aw, Y. Takenaga^aw,

†Deceased

1Also at JINR, Dubna, Russia

2Also at Institute of Particle Physics, Canada

3Also at BMCC/CUNY, New York, New York, U.S.A.

Y. Takeuchi^aa, H.A. Tanaka^e,2, K. Tanaka^r, M. Tanaka^r, M.M. Tanaka^r, N. Tanimoto^ax, K. Tashiro^ai, I.J. Taylor^ah,u, A. Terashima^r, D. Terhorst^ap, R. Terri^am, L.F. Thompson^ar, A. Thorley^ad, M. Thorpe^au, W. Toki^k,ah, T. Tomaru^r, Y. Totsuka^r,†, C. Touramanis^ad, T. Tsukamoto^r, V. Tvaskis^ba, M. Tzanov^ae,j, Y. Uchida^u, K. Ueno^aw, M. Usseglio^h, A. Vacheret^u, M. Vagins^g, J.F. Van Schalkwyk^u,

J.-C. Vanelⁿ, G. Vasseur^h, O. Veledar^ar, P. Vincent^az, T. Wachala^q, A.V. Waldron^aj, C.W. Walter^m, P.J. Wanderer^f, M.A. Ward^ar, G.P. Ward^ar, D. Wark^au,u, D. Warner^k, M.O. Wascko^u, A. Weber^aj,au, R. Wendell^m, J. Wendland^e, N. West^aj, L.H. Whitehead^bd,

G. Wikstr¨om^p, R.J. Wilkes^be, M.J. Wilking^az, Z. Williamson^aj, J.R. Wilson^am, R.J. Wilson^k, K. Wong^az, T. Wongjirad^m, S. Yamada^aw, Y. Yamada^r, A. Yamamoto^r, K. Yamamoto^ai, Y. Yamanoi^r, H. Yamaoka^r, C. Yanagisawa^ah,3, T. Yano^aa, S. Yen^az, N. Yershov^z, M. Yokoyama^av, A. Zalewska^q, J. Zalipska^e, K. Zaremba^bc, M. Ziembicki^bc, E.D. Zimmerman^j, M. Zito^h, J. Zmuda^bf

(The T2K Collaboration)

aUniversity of Alberta, Centre for Particle Physics, Department of Physics, Edmonton, Alberta, Canada

bThe Andrzej Soltan Institute for Nuclear Studies, Warsaw, Poland

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

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

eUniversity of British Columbia, Department of Physics and Astronomy, Vancouver, British Columbia, Canada

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

gUniversity of California, Irvine, Department of Physics and Astronomy, Irvine, California, U.S.A.

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

iChonnam National University, Department of Physics, Kwangju, Korea

jUniversity of Colorado at Boulder, Department of Physics, Boulder, Colorado, U.S.A.

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

lDongshin University, Department of Physics, Naju, Korea

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

nEcole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, Palaiseau, France

oETH Zurich, Institute for Particle Physics, Zurich, Switzerland

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

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

rHigh Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki, Japan

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

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

uImperial College London, Department of Physics, London, United Kingdom

vINFN Sezione di Bari and Universit`a e Politecnico di Bari, Dipartimento Interuniversitario di Fisica, Bari, Italy

wINFN Sezione di Napoli and Universit`a di Napoli, Dipartimento di Fisica, Napoli, Italy

xINFN Sezione di Padova and Universit`a di Padova, Dipartimento di Fisica, Padova, Italy

yINFN Sezione di Roma and Universit`a di Roma “La Sapienza”, Roma, Italy

zInstitute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia

aaKobe University, Kobe, Japan

abKyoto University, Department of Physics, Kyoto, Japan

acLancaster University, Physics Department, Lancaster, United Kingdom

adUniversity of Liverpool, Department of Physics, Liverpool, United Kingdom

aeLouisiana State University, Department of Physics and Astronomy, Baton Rouge, Louisiana, U.S.A.

afUniversit´e de Lyon, Universit´e Claude Bernard Lyon 1, IPN Lyon (IN2P3), Villeurbanne, France

agMiyagi University of Education, Department of Physics, Sendai, Japan

ahState University of New York at Stony Brook, Department of Physics and Astronomy, Stony Brook, New York, U.S.A.

aiOsaka City University, Department of Physics, Osaka, Japan

ajOxford University, Department of Physics, Oxford, United Kingdom

akUPMC, Universit´e Paris Diderot, CNRS/IN2P3, Laboratoire de Physique Nucl´eaire et de Hautes Energies (LPNHE), Paris, France

alUniversity of Pittsburgh, Department of Physics and Astronomy, Pittsburgh, Pennsylvania, U.S.A.

amQueen Mary University of London, School of Physics, London, United Kingdom

anUniversity of Regina, Physics Department, Regina, Saskatchewan, Canada

aoUniversity of Rochester, Department of Physics and Astronomy, Rochester, New York, U.S.A.

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

aqSeoul National University, Department of Physics, Seoul, Korea

arUniversity of Sheffield, Department of Physics and Astronomy, Sheffield, United Kingdom

asUniversity of Silesia, Institute of Physics, Katowice, Poland

atSTFC, Daresbury Laboratory, Warrington, United Kingdom

auSTFC, Rutherford Appleton Laboratory, Harwell Oxford, United Kingdom

avUniversity of Tokyo, Department of Physics, Tokyo, Japan

awUniversity of Tokyo, Institute for Cosmic Ray Research, Kamioka Observatory, Kamioka, Japan

axUniversity of Tokyo, Institute for Cosmic Ray Research, Research Center for Cosmic Neutrinos, Kashiwa, Japan

ayUniversity of Toronto, Department of Physics, Toronto, Ontario, Canada

azTRIUMF, Vancouver, British Columbia, Canada

baUniversity of Victoria, Department of Physics and Astronomy, Victoria, British Columbia, Canada

bbUniversity of Warsaw, Faculty of Physics, Warsaw, Poland

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

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

beUniversity of Washington, Department of Physics, Seattle, Washington, U.S.A.

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

bgYork University, Department of Physics and Astronomy, Toronto, Ontario, Canada

Abstract

The T2K experiment is a long-baseline neutrino oscillation experiment. Its main goal is to measure the last unknown lepton sector mixing angle θ13 by observingνe appearance in aνµbeam. It also aims to make a precision measurement of the known oscillation parameters,∆m²23 and sin²2θ23, viaνµdisappearance studies. Other goals of the experiment include various neutrino cross section measurements and sterile neutrino searches. The experiment uses an intense proton beam generated by the J-PARC accelerator in Tokai, Japan, and is composed of a neutrino beamline, a near detector complex (ND280), and a far detector (Super- Kamiokande) located 295 km away from J-PARC. This paper provides a comprehensive review of the instrumentation aspect of the T2K experiment and a summary of the vital information for each subsystem.

Keywords: Neutrinos, Neutrino Oscillation, Long Baseline, T2K, J-PARC, Super-Kamiokande PACS:14.60.Lm, 14.60.Pq, 29.20.dk, 29.40.Gx, 29.40.Ka, 29.40.Mc, 29.40.Vj, 29.40.Wk, 29.85.Ca

1. Introduction

The T2K (Tokai-to-Kamioka) experiment [1] is a long base- line neutrino oscillation experiment designed to probe the mix- ing of the muon neutrino with other species and shed light on the neutrino mass scale. It is the first long baseline neutrino os- cillation experiment proposed and approved to look explicitly for the electron neutrino appearance from the muon neutrino, thereby measuring θ13, the last unknown mixing angle in the lepton sector.

T2K’s physics goals include the measurement of the neutrino oscillation parameters with precision ofδ(∆m²23) ∼ 10⁻⁴eV² and δ(sin²2θ23) ∼ 0.01 via ν_µ disappearance studies, and achieving a factor of about 20 better sensitivity compared to the current best limit on θ13 from the CHOOZ experi- ment [2] through the search forνµ→νeappearance (sin²2θµe'

1

2sin²2θ13>0.004 at 90% CL for CP violating phaseδ=0). In addition to neutrino oscillation studies, the T2K neutrino beam (withE_ν ∼1 GeV) will enable a rich fixed-target physics pro- gram of neutrino interaction studies at energies covering the transition between the resonance production and deep inelastic scattering regimes.

T2K uses Super-Kamiokande [3] as the far detector to mea- sure neutrino rates at a distance of 295 km from the accelerator, and near detectors to sample the beam just after production.

The experiment includes a neutrino beamline and a near de- tector complex at 280 m (ND280), both of which were newly constructed. Super-Kamiokande was upgraded and restored to 40% photocathode coverage (the same as the original Super- Kamiokande detector) with new photomultiplier tubes in 2005–

06, following the accident of 2001. Fig. 1 shows a schematic layout of the T2K experiment as a whole.

T2K adopts the off-axis method [4] to generate the narrow- band neutrino beam using the new MW-class proton syn- chrotron at J-PARC⁴. In this method the neutrino beam is pur-

4Japan Proton Accelerator Research Complex jointly constructed and oper-

295 km

280 m J-PARC Near Detector Super-Kamiokande

1000 m

Neutrino Beam

Figure 1: A schematic of a neutrino’s journey from the neu- trino beamline at J-PARC, through the near detectors (green dot) which are used to determine the properties of the neutrino beam, and then 295 km underneath the main island of Japan to Super-Kamiokande.

posely directed at an angle with respect to the baseline connect- ing the proton target and the far detector, Super-Kamiokande.

The off-axis angle is set at 2.5^◦so that the narrow-band muon- neutrino beam generated toward the far detector has a peak energy at ∼0.6 GeV, which maximizes the effect of the neu- trino oscillation at 295 km and minimizes the background to electron-neutrino appearance detection. The angle can be re- duced to 2.0^◦, allowing variation of the peak neutrino energy, if necessary.

The near detector site at ∼280 m from the production tar- get houses on-axis and off-axis detectors. The on-axis detec- tor (INGRID), composed of an array of iron/scintillator sand- wiches, measures the neutrino beam direction and profile. The off-axis detector, immersed in a magnetic field, measures the muon neutrino flux and energy spectrum, and intrinsic electron neutrino contamination in the beam in the direction of the far detector, along with measuring rates for exclusive neutrino re- actions. These measurements are essential in order to charac- terize signals and backgrounds that are observed in the Super- Kamiokande far detector.

ated by KEK and JAEA.

The off-axis detector is composed of: a water-scintillator de- tector optimized to identifyπ⁰’s (the PØD); the tracker consist- ing of time projection chambers (TPCs) and fine grained de- tectors (FGDs) optimized to study charged current interactions;

and an electromagnetic calorimeter (ECal) that surrounds the PØD and the tracker. The whole off-axis detector is placed in a 0.2 T magnetic field provided by the recycled UA1 mag- net, which also serves as part of a side muon range detector (SMRD).

The far detector, Super-Kamiokande, is located in the Mozumi mine of the Kamioka Mining and Smelting Company, near the village of Higashi-Mozumi, Gifu, Japan. The detector cavity lies under the peak of Mt. Ikenoyama, with 1000 m of rock, or 2700 meters-water-equivalent (m.w.e.) mean overbur- den. It is a water Cherenkov detector consisting of a welded stainless-steel tank, 39 m in diameter and 42 m tall, with a total nominal water capacity of 50,000 tons. The detector contains approximately 13,000 photomultiplier tubes (PMTs) that im- age neutrino interactions in pure water. Super-Kamiokande has been running since 1996 and has had four distinctive running periods. The latest period, SK-IV, is running stably and fea- tures upgraded PMT readout electronics. A detailed description of the detector can be found elsewhere [3].

Construction of the neutrino beamline started in April 2004.

The complete chain of accelerator and neutrino beamline was successfully commissioned during 2009, and T2K began ac- cumulating neutrino beam data for physics analysis in January 2010.

Construction of the majority of the ND280 detectors was completed in 2009 with the full installation of INGRID, the central ND280 off-axis sub-detectors (PØD, FGD, TPC and downstream ECal) and the SMRD. The ND280 detectors be- gan stable operation in February 2010. The rest of the ND280 detector (the ECals) was completed in the fall of 2010.

The T2K collaboration consists of over 500 physicists and technical staff members from 59 institutions in 12 countries (Canada, France, Germany, Italy, Japan, Poland, Russia, South Korea, Spain, Switzerland, the United Kingdom and the United States).

This paper provides a comprehensive review of the instru- mentation aspect of the T2K experiment and a summary of the vital information for each subsystem. Detailed descriptions of some of the major subsystems, and their performance, will be presented in separate technical papers.

2. J-PARC Accelerator

J-PARC, which was newly constructed at Tokai, Ibaraki, con- sists of three accelerators [5]: a linear accelerator (LINAC), a rapid-cycling synchrotron (RCS) and the main ring (MR) synchrotron. An H⁻ beam is accelerated up to 400 MeV⁵ (181 MeV at present) by the LINAC, and is converted to an H⁺ beam by charge-stripping foils at the RCS injection. The

5Note that from here on all accelerator beam energies given are kinetic en- ergies.

Table 1: Machine design parameters of the J-PARC MR for the fast extraction.

Circumference 1567 m

Beam power ∼750 kW

Beam kinetic energy 30 GeV Beam intensity ∼3×10¹⁴p/spill

Spill cycle ∼0.5 Hz

Number of bunches 8/spill RF frequency 1.67 – 1.72 MHz

Spill width ∼5µsec

beam is accelerated up to 3 GeV by the RCS with a 25 Hz cy- cle. The harmonic number of the RCS is two, and there are two bunches in a cycle. About 5% of these bunches are supplied to the MR. The rest of the bunches are supplied to the muon and neutron beamline in the Material and Life Science Facility. The proton beam injected into the MR is accelerated up to 30 GeV.

The harmonic number of the MR is nine, and the number of bunches in the MR is eight (six before June 2010). There are two extraction points in the MR: slow extraction for the hadron beamline and fast extraction for the neutrino beamline.

In the fast extraction mode, the eight circulating proton bunches are extracted within a single turn by a set of five kicker magnets. The time structure of the extracted proton beam is key to discriminating various backgrounds, including cosmic rays, in the various neutrino detectors. The parameters of the J-PARC MR for the fast extraction are listed in Tab. 1.

3. T2K Neutrino Beamline

Each proton beam spill consists of eight proton bunches ex- tracted from the MR to the T2K neutrino beamline, which pro- duces the neutrino beam.

The neutrino beamline is composed of two sequential sec- tions: the primary and secondary beamlines. In the primary beamline, the extracted proton beam is transported to point to- ward Kamioka. In the secondary beamline, the proton beam impinges on a target to produce secondary pions, which are fo- cused by magnetic horns and decay into neutrinos. An overview of the neutrino beamline is shown in Fig. 2. Each component of the beamline is described in this section.

The neutrino beamline is designed so that the neutrino energy spectrum at Super-Kamiokande can be tuned by changing the off-axis angle down to a minimum of∼2.0^◦, from the current (maximum) angle of∼2.5^◦. The unoscillatedνµflux at Super- Kamiokande with this off-axis angle is shown in Fig. 3. Precise measurements of the baseline distance and off-axis angle were determined by a GPS survey, described in Section 3.6.1.

3.1. Primary Beamline

The primary beamline consists of the preparation section (54 m long), arc section (147 m) and final focusing section (37 m). In the preparation section, the extracted proton beam

0 50 100 m

Main Ring

Secondary beamline

(1) Preparation section (2) Arc section (3) Final focusing section (4) Target station (5) Decay volume (6) Beam dump

ND280

(1) (2) (3) (5) (4)

(6)

Figure 2: Overview of the T2K neutrino beamline.

(GeV) Eν

0 0.5 1 1.5 2 2.5 3 3.5

]2 POT/50 MeV/cm21 Flux[/10

0 0.2 0.4 0.6 0.8 1 1.2

106

×

Figure 3: The unoscillatedν_µflux at Super-Kamiokande with an off-axis angle of 2.5^◦ when the electromagnetic horns are operated at 250 kA.

is tuned with a series of 11 normal conducting magnets (four steering, two dipole and five quadrupole magnets) so that the beam can be accepted by the arc section. In the arc section, the beam is bent toward the direction of Kamioka by 80.7^◦, with a 104 m radius of curvature, using 14 doublets of supercon- ducting combined function magnets (SCFMs) [6, 7, 8]. There are also three pairs of horizontal and vertical superconducting steering magnets to correct the beam orbit. In the final focus- ing section, ten normal conducting magnets (four steering, two dipole and four quadrupole magnets) guide and focus the beam onto the target, while directing the beam downward by 3.637^◦ with respect to the horizontal.

A well-tuned proton beam is essential for stable neutrino beam production, and to minimize beam loss in order to achieve high-power beam operation. Therefore, the intensity, position, profile and loss of the proton beam in the primary sections are precisely monitored by five current transformers (CTs), 21 elec- trostatic monitors (ESMs), 19 segmented secondary emission monitors (SSEMs) and 50 beam loss monitors (BLMs), respec-

Figure 4: Photographs of the primary beamline monitors. Up- per left: CT. Upper right: ESM. Lower left: SSEM. Lower right: BLM.

Figure 5: Location of the primary beamline monitors.

tively. Photographs of the monitors are shown in Fig. 4, while the monitor locations are shown in Fig. 5. Polyimide cables and ceramic feedthroughs are used for the beam monitors, because of their radiation tolerance.

The beam pipe is kept at∼3×10⁻⁶Pa using ion pumps, in or- der to be connected with the beam pipe of the MR and to reduce the heat load to the SCFMs. The downstream end of the beam pipe is connected to the “monitor stack”: the 5 m tall vacuum vessel embedded within the 70 cm thick wall between the pri- mary beamline and secondary beamline. The most downstream ESM and SSEM are installed in the monitor stack. Because of the high residual radiation levels, the monitor stack is equipped with a remote-handling system for the monitors.

3.1.1. Normal Conducting Magnet

The normal conducting magnets are designed to be tolerant of radiation and to be easy to maintain in the high-radiation environment. For the four most upstream magnets in the prepa- ration section, a mineral insulation coil is used because of its radiation tolerance. To minimize workers’ exposure to radia-

tion, remote maintenance systems are installed such as twist- lock slings, alignment dowel pins, and quick connectors for cooling pipes and power lines.

For the quadrupole magnets, “flower-shaped” beam pipes, whose surfaces were made in the shape of the magnetic pole surface, are adopted to maximize their apertures.

3.1.2. Superconducting Combined Function Magnet (SCFM) In total, there are 28 SCFMs [9, 10, 11, 12], each with a coil aperture of 173.4 mm. The operating current for a 30 GeV pro- ton beam is 4360 A, while the magnets themselves were tested up to 7500 A, which corresponds to a 50 GeV proton beam.

The combined field is generated with a left-right asymmet- ric single layer Rutherford-type coil, made of NbTi/Cu. Two SCFMs are enclosed in one cryostat in forward and backward directions to constitute a defocus-focus doublet, while each dipole field is kept in the same direction. All the SCFMs are cooled in series with supercritical helium at 4.5 K and are ex- cited with a power supply (8 kA, 10 V).

There are also three superconducting corrector dipole mag- nets, which are cooled by conduction, in the SCFM section.

Each magnet has two windings, one for vertical and one for horizontal deflections. These magnets allow the beam to be precisely positioned along the beamline (to minimize losses).

The magnet safety system (MSS) protects the magnets and the bus-bars of the primary beamline in the case of an abnor- mal condition, and supplements the passive safety protection provided by cold diodes mounted in parallel with the supercon- ducting magnets. The MSS is based on the detection of a re- sistive voltage difference across the magnet that would appear in the case of a quench. It then secures the system by shutting down the magnet power supply and issuing a beam abort inter- lock signal. Most units of the MSS are dual redundant. This redundancy increases the reliability of the system. The MSS is based on 33 MD200 boards [13].

3.1.3. Beam Intensity Monitor

Beam intensity is measured with five current transformers (CTs). Each CT is a 50-turn toroidal coil around a cylindrical ferromagnetic core. To achieve high-frequency response up to 50 MHz for the short-pulsed bunches and to avoid saturation caused by a large peak current of 200 A, CTs use a FINEMET^® (nanocrystalline Fe-based soft magnetic material) core, which has a high saturation flux density, high relative permeability and low core loss over a wide frequency range. The core’s inner diameter is 260 mm, its outer diameter is 340 mm and it has a mass of 7 kg. It is impregnated with epoxy resin. To achieve high radiation hardness, polyimide tape and alumina fiber tape are used to insulate the core and wire. Each CT is covered by an iron shield to block electromagnetic noise.

Each CT’s signal is transferred through about 100 m of 20D colgate cable and read by a 160 MHz Flash ADC (FADC). The CT is calibrated using another coil around the core, to which a pulse current, shaped to emulate the passage of a beam bunch, is applied. The CT measures the absolute proton beam intensity with a 2% uncertainty and the relative intensity with a 0.5%

fluctuation. It also measures the beam timing with precision better than 10 ns.

3.1.4. Beam Position Monitor

Each electrostatic monitor (ESM) has four segmented cylin- drical electrodes surrounding the proton beam orbit (80^◦ cov- erage per electrode). By measuring top-bottom and left-right asymmetry of the beam-induced current on the electrodes, it monitors the proton beam center position nondestructively (without direct interaction with the beam).

The longitudinal length of an ESM is 125 mm for the 15 ESMs in the preparation and final focusing sections, 210 mm for the five ESMs in the arc section and 160 mm for the ESM in the monitor stack. The signal from each ESM is read by a 160 MHz FADC.

The measurement precision of the beam position is less than 450 µm (20–40 µm for the measurement fluctuation, 100–

400µm for the alignment precision and 200µm for the system- atic uncertainty other than the alignment), while the require- ment is 500µm.

3.1.5. Beam Profile Monitor

Each segmented secondary emission monitor (SSEM) has two thin (5µm, 10⁻⁵interaction lengths) titanium foils stripped horizontally and vertically, and an anode HV foil between them.

The strips are hit by the proton beam and emit secondary elec- trons in proportion to the number of protons that go through the strip. The electrons drift along the electric field and induce currents on the strips. The induced signals are transmitted to 65 MHz FADCs through twisted-pair cables. The proton beam profile is reconstructed from the corrected charge distribution on a bunch-by-bunch basis. The strip width of each SSEM ranges from 2 to 5 mm, optimized according to the expected beam size at the installed position. The systematic uncertainty of the beam width measurement is 200µm while the require- ment is 700µm. Optics parameters of the proton beam (Twiss parameters and emittance) are reconstructed from the profiles measured by the SSEMs, and are used to estimate the profile center, width and divergence at the target.

Since each SSEM causes beam loss (0.005% loss), they are remotely inserted into the beam orbit only during beam tuning, and extracted from the beam orbit during continuous beam op- eration.

3.1.6. Beam Loss Monitor

To monitor the beam loss, 19 and 10 BLMs are installed near the beam pipe in the preparation and final focusing sections re- spectively, while 21 BLMs are positioned near the SCFMs in the arc section. Each BLM (Toshiba Electron Tubes & Devices E6876-400) is a wire proportional counter filled with an Ar- CO₂mixture [14].

The signal is integrated during the spill and if it exceeds a threshold, a beam abort interlock signal is fired. The raw signal before integration is read by the FADCs with 30 MHz sampling for the software monitoring.

By comparing the beam loss with and without the SSEMs in the beamline, it was shown that the BLM has a sensitivity

Target station

Beam dump

(1) (2)

(3)

(4) (5)

(6)

Muon monitor (1) Beam window (2) Baffle (3) OTR (4) Target and

first horn (5) Second horn (6) Third horn

Figure 6: Side view of the secondary beamline. The length of the decay volume is∼96 m.

down to a 16 mW beam loss. In the commissioning run, it was confirmed that the residual dose and BLM data integrated during the period have good proportionality. This means that the residual dose can be monitored by watching the BLM data.

3.2. Secondary Beamline

Produced pions decay in flight inside a single volume of

∼1500 m³, filled with helium gas (1 atm) to reduce pion ab- sorption and to suppress tritium and NO_x production by the beam. The helium vessel is connected to the monitor stack via a titanium-alloy beam window which separates the vacuum in the primary beamline and the helium gas volume in the secondary beamline. Protons from the primary beamline are directed to the target via the beam window.

The secondary beamline consists of three sections: the target station, decay volume and beam dump (Fig. 6). The target sta- tion contains: a baffle which is a collimator to protect the mag- netic horns; an optical transition radiation monitor (OTR) to monitor the proton beam profile just upstream of the target; the target to generate secondary pions; and three magnetic horns excited by a 250 kA (designed for up to 320 kA) current pulse to focus the pions. The produced pions enter the decay vol- ume and decay mainly into muons and muon neutrinos. All the hadrons, as well as muons below∼5 GeV/c, are stopped by the beam dump. The neutrinos pass through the beam dump and are used for physics experiments. Any muons above∼5 GeV/c that also pass through the beam dump are monitored to characterize the neutrino beam.

3.2.1. Target Station

The target station consists of the baffle, OTR, target, and horns, all located inside a helium vessel. The target station is separated from the primary beamline by a beam window at the upstream end, and is connected to the decay volume at the downstream end.

The helium vessel, which is made of 10 cm thick steel, is 15 m long, 4 m wide and 11 m high. It is evacuated down to 50 Pa before it is filled with helium gas. Water cooling chan- nels, called plate coils, are welded to the surface of the vessel, and∼30^◦C water cools the vessel to prevent its thermal defor- mation. An iron shield with a thickness of∼2 m and a concrete shield with a thickness of∼1 m are installed above the horns inside the helium vessel. Additionally,∼4.5 m thick concrete shields are installed above the helium vessel.

The equipment and shields inside the vessel are removable by remote control in case of maintenance or replacement of the horns or target. Beside the helium vessel, there is a maintenance area where manipulators and a lead-glass window are installed, as well as a depository for radio-activated equipment.

3.2.2. Beam Window

The beam window, comprising two helium-cooled 0.3 mm thick titanium-alloy skins, separates the primary proton beam- line vacuum from the target station. The beam window assem- bly is sealed both upstream and downstream by inflatable bel- lows vacuum seals to enable it to be removed and replaced if necessary.

3.2.3. Baffle

The baffle is located between the beam window and OTR. It is a 1.7 m long, 0.3 m wide and 0.4 m high graphite block, with a beam hole of 30 mm in diameter. The primary proton beam goes through this hole. It is cooled by water cooling pipes.

3.2.4. Optical Transition Radiation Monitor

The OTR has a thin titanium-alloy foil, which is placed at 45^◦ to the incident proton beam. As the beam enters and exits the foil, visible light (transition radiation) is produced in a narrow cone around the beam. The light produced at the entrance tran- sition is reflected at 90^◦to the beam and directed away from the target area. It is transported in a dogleg path through the iron and concrete shielding by four aluminum 90^◦off-axis parabolic mirrors to an area with lower radiation levels. It is then col- lected by a charge injection device camera to produce an image of the proton beam profile.

The OTR has an eight-position carousel holding four titan- ium-alloy foils, an aluminum foil, a fluorescent ceramic foil of 100µm thickness, a calibration foil and an empty slot (Fig. 7).

A stepping motor is used to rotate the carousel from one foil to the next. The aluminum (higher reflectivity than titanium) and ceramic (which produces fluorescent light with higher in- tensity than OTR light) foils are used for low and very low in- tensity beam, respectively. The calibration foil has precisely machined fiducial holes, of which an image can be taken us- ing back-lighting from lasers and filament lights. It is used for monitoring the alignment of the OTR system. The empty slot allows back-lighting of the mirror system to study its transport efficiency.

3.2.5. Target

The target core is a 1.9 interaction length (91.4 cm long), 2.6 cm diameter and 1.8 g/cm³ graphite rod. If a material sig-

nificantly denser than graphite were used for the target core, it would be melted by the pulsed beam heat load.

The core and a surrounding 2 mm thick graphite tube are sealed inside a titanium case which is 0.3 mm thick. The tar- get assembly is supported as a cantilever inside the bore of the first horn inner conductor with a positional accuracy of 0.1 mm.

The target is cooled by helium gas flowing through the gaps be- tween the core and tube and between the tube and case. For the 750 kW beam, the flow rate is∼32 g/s helium gas with a helium outlet pressure of 0.2 MPa, which corresponds to a flow speed of∼250 m/s. When the 750 kW proton beam interacts with the target, the temperature at the center is expected to reach 700^◦C, using the conservative assumption that radiation damage has reduced the thermal conductivity of the material by a factor of four.

The radiation dose due to the activation of the target is esti- mated at a few Sv/h six months after a one year’s irradiation by the 750 kW beam [15].

3.2.6. Magnetic Horn

The T2K beamline uses three horns. Each magnetic horn consists of two coaxial (inner and outer) conductors which en- compass a closed volume [16, 17]. A toroidal magnetic field is generated in that volume. The field varies as 1/r, whereris the distance from the horn axis. The first horn collects the pions which are generated at the target installed in its inner conduc- tor. The second and third horns focus the pions. When the horn is run with a operation current of 320 kA, the maximum field is 2.1 T and the neutrino flux at Super-Kamiokande is increased by a factor of∼16 (compared to horns at 0 kA) at the spectrum peak energy (∼0.6 GeV).

The horn conductor is made of aluminum alloy (6061-T6).

The horns’ dimensions (minimum inside diameter, inner con- ductor thickness, outside diameter and length respectively) are 54 mm, 3 mm, 400 mm and 1.5 m for the first horn, 80 mm, 3 mm, 1000 mm and 2 m for the second horn, and 140 mm, 3 mm, 1400 mm and 2.5 m for the third horn. They are opti- mized to maximize the neutrino flux; the inside diameter is as small as possible to achieve the maximum magnetic field, and the conductor is as thin as possible to minimize pion absorption while still being tolerant of the Lorentz force, created from the 320 kA current and the magnetic field, and the thermal shock from the beam.

The pulse current is applied via a pulse transformer with a turn ratio of 10:1, which is installed beside the helium vessel in the target station. The horns are connected to the secondary side of the pulse transformer in series using aluminum bus-bars.

The currents on the bus-bars are monitored by four Rogowski coils per horn with a 200 kHz FADC readout. The measure- ment uncertainty of the absolute current is less than∼2%. The horn magnetic field was measured with a Hall probe before in- stallation, and the uncertainty of the magnetic field strength is approximately 2% for the first horn and less than 1% for the second and third horns.

Figure 7: Top: Photograph of the OTR carousel. Bottom:

Cross section of the first horn and target.

3.2.7. Decay Volume

The decay volume is a∼96 m long steel tunnel. The cross section is 1.4 m wide and 1.7 m high at the upstream end, and 3.0 m wide and 5.0 m high at the downstream end. The decay volume is surrounded by 6 m thick reinforced concrete shield- ing. Along the beam axis, 40 plate coils are welded on the steel wall, whose thickness is 16 mm, to cool the wall and concrete to below 100^◦C using water.

3.2.8. Beam Dump

The beam dump sits at the end of the decay volume. The distance between the center of the target and the upstream sur- face of the beam dump along the neutrino beam direction for the off-axis angle of 2.5^◦ is 109 m. The beam dump’s core is made of 75 tons of graphite (1.7 g/cm³), and is 3.174 m long, 1.94 m wide and 4.69 m high. It is contained in the helium ves- sel. Fifteen iron plates are placed outside the vessel and two inside, at the downstream end of the graphite core, to give a total iron thickness of 2.40 m. Only muons above∼5.0 GeV/c can go through the beam dump to reach the downstream muon pit.

The core is sandwiched on both sides by aluminum cooling modules which contain water channels. The temperature in the center of the core is kept at around 150^◦C for the 750 kW beam.

3.3. Muon Monitor

The neutrino beam intensity and direction can be monitored on a bunch-by-bunch basis by measuring the distribution pro- file of muons, because muons are mainly produced along with neutrinos from the pion two-body decay. The neutrino beam direction is determined to be the direction from the target to the center of the muon profile. The muon monitor [18, 19] is located just behind the beam dump. The muon monitor is de- signed to measure the neutrino beam direction with a precision better than 0.25 mrad, which corresponds to a 3 cm precision of the muon profile center. It is also required to monitor the stability of the neutrino beam intensity with a precision better than 3%.

A detector made of nuclear emulsion was installed just down- stream of the muon monitor to measure the absolute flux and momentum distribution of muons.

3.3.1. Characteristics of the Muon Flux

Based on the beamline simulation package, described in Sec- tion 3.5, the intensity of the muon flux at the muon monitor, for 3.3×10¹⁴ protons/spill and 320 kA horn current, is estimated to be 1×10⁷charged particles/cm²/bunch with a Gaussian-like profile around the beam center and approximately 1 m in width.

The flux is composed of around 87% muons, with delta-rays making up the remainder.

3.3.2. Muon Monitor Detectors

The muon monitor consists of two types of detector arrays:

ionization chambers at 117.5 m from the target and silicon PIN photodiodes at 118.7 m (Fig. 8). Each array holds 49 sensors at 25 cm×25 cm intervals and covers a 150×150 cm²area.

The collected charge on each sensor is read out by a 65 MHz FADC. The 2D muon profile is reconstructed in each array from the distribution of the observed charge.

The arrays are fixed on a support enclosure for thermal insu- lation. The temperature inside the enclosure is kept at around 34^◦C (within±0.7^◦C variation) with a sheathed heater, as the signal gain in the ionization chamber is dependent on the gas temperature.

An absorbed dose at the muon monitor is estimated to be about 100 kGy for a 100-day operation at 750 kW. Therefore, every component in the muon pit is made of radiation-tolerant and low-activation material such as polyimide, ceramic, or alu- minum.

3.3.3. Ionization Chamber

There are seven ionization chambers, each of which contains seven sensors in a 150×50×1956 mm³aluminum gas tube. The 75×75×3 mm³active volume of each sensor is made by two parallel plate electrodes on alumina-ceramic plates. Between the electrodes, 200 V is applied.

Two kinds of gas are used for the ionization chambers ac- cording to the beam intensity: Ar with 2% N2for low intensity, and He with 1% N2for high intensity. The gas is fed in at ap- proximately 100 cm³/min. The gas temperature, pressure and oxygen contamination are kept at around 34^◦C with a 1.5^◦C

Figure 8: Photograph of the muon monitor inside the support enclosure. The silicon PIN photodiode array is on the right side and the ionization chamber array is on the left side. The muon beam enters from the left side.

gradient and±0.2^◦C variation, at 130±0.2 kPa (absolute), and below 2 ppm, respectively.

3.3.4. Silicon PIN Photodiode

Each silicon PIN photodiode (Hamamatsu^® S3590-08) has an active area of 10×10 mm²and a depletion layer thickness of 300µm. To fully deplete the silicon layer, 80 V is applied.

The intrinsic resolution of the muon monitor is less than 0.1% for the intensity and less than 0.3 cm for the profile center.

3.3.5. Emulsion Tracker

The emulsion trackers are composed of two types of mod- ules. The module for the flux measurement consists of eight consecutive emulsion films [20]. It measures the muon flux with a systematic uncertainty of 2%. The other module for the momentum measurement is made of 25 emulsion films inter- leaved by 1 mm lead plates, which can measure the momentum of each particle by multiple Coulomb scattering with a preci- sion of 28% at a muon energy of 2 GeV/c [21, 22]. These films are analyzed by scanning microscopes [23, 24].

3.4. Beamline Online System

For the stable and safe operation of the beamline, the online system collects information on the beamline equipment and the beam measured by the beam monitors, and feeds it back to the operators. It also provides Super-Kamiokande with the spill information for event synchronization by means of GPS, which is described in detail in Section 3.6.2.

3.4.1. DAQ System

The signals from each beam monitor are brought to one of five front-end stations in different buildings beside the beam- line. The SSEM, BLM, and horn current signals are digitized by a 65 MHz FADC in the COPPER system [25]. The CT and ESM signals are digitized by a 160 MHz VME FADC [26].

The GPS for the event synchronization and the OTR both use custom-made readout electronics. All of these readout systems are managed by the MIDAS framework [27], and the event builder records fully concatenated events every spill, before the next spill is issued. MIDAS’s event monitoring system locks the internal data holding buffer. To minimize the locking time, which can have a negative effect on the DAQ system’s response time, an event distributor was developed. It receives event data from the MIDAS server and distributes the data in ROOT for- mat to the clients. No reduction is applied to the output from the ADCs and the event data size remains constant at 1.6 MB.

3.4.2. Beamline Control System

Information on the beamline (the beam monitor outputs, spill number and status of the beamline equipment) is recorded by EPICS [28]. EPICS also controls the beamline equipment using programmable logic controllers (PLCs).

Based on the data from EPICS, the beam orbit and optics are simulated by SAD [29], and the magnet currents to be adjusted are also calculated.

3.4.3. Interlock

The function of the interlock system is to protect people (PPS: person protection system) and the machines (MPS: ma- chine protection system). The PPS can be fired by an emer- gency stop button, or safety sensors such as door interlocks and radiation monitors. The MPS can be fired by a quenching of the SCFMs, an error from the normal conducting magnet or horn system, an excess in the loss monitor signal, or other machine- related causes.

3.5. Beamline Simulation for Neutrino Flux Estimation The neutrino flux is predicted by a Monte Carlo simulation based on experimental data. Specifically, hadron production by 30 GeV protons on a graphite target was measured by a ded- icated experiment, NA61/SHINE [30, 31], which fully covers the kinematic region of interest for T2K.

In the beam MC, the detailed geometry of the secondary beamline is described in the code. Protons with a kinetic energy of 30 GeV are injected into the graphite target and then sec- ondary particles are produced and focused in the horn magnets.

The secondaries and any un-interacted protons are tracked until they decay into neutrinos or are stopped at the beam dump. The tracks of neutrinos are extrapolated to the near and far detec- tors, providing the predicted fluxes and energy spectra at both detector sites.

The primary interaction of the 30 GeV proton with car- bon is simulated based on NA61/SHINE data. Other hadronic interactions inside the target are simulated by FLUKA [32].

The interactions outside the target are simulated using GEANT3/GCALOR [33] with the interaction cross sections tuned to experimental data.

3.6. Global Alignment and Time Synchronization 3.6.1. Global Position Survey and Alignment

In a long baseline neutrino experiment, controlling the direc- tion of the neutrino beam is one of the most important aspects.

For the T2K neutrino experiment, it is necessary to consider the three-dimensional geometry of the earth, since it covers a distance of∼300 km from J-PARC to Super-Kamiokande. De- termining the correct direction is not simple. Therefore, surveys were performed, including a long baseline GPS survey between Tokai and Kamioka.

Based on the surveys, the primary beamline components, tar- get, and horns were aligned in order to send the neutrino beam in the right direction. The muon monitor and the neutrino near detectors were also aligned in order to monitor the neutrino beam direction. A good alignment of the components is also necessary in order to reduce irradiation in a high-intensity pro- ton beamline.

A complete neutrino beamline survey is carried out on a yearly basis. There are five penetration holes in the neutrino beamline to connect a ground survey network with an under- ground survey; one at the preparation section, two at the final focusing section and two at the muon pit. In the target station, the underground survey points were transferred to the ground level so that they can be monitored even after the underground helium vessel is closed. In the primary beamline, a survey and alignment is carried out using a laser tracker with a spatial res- olution of 50µm at a distance shorter than 20 m. The super- conducting magnets were aligned to better than 100µm and the normal conducting quadrupole magnets were aligned to better than 1 mm. In the other places, a survey is carried out using a total station which gives a spatial resolution of about 2 mm at a distance of 100 m.

We observed a ground sink of a few tens of millimeters dur- ing the construction stage. It was taken into account at the in- stallation of the beamline components. After the installation at the primary beamline, we still observed a sink of several millimeters at the final focusing section and the target station.

Therefore the beam was tuned to follow the beamline sink.

The required directional accuracy from the physics point of view is 1×10⁻³rad. The directional accuracy of a long baseline GPS survey is several times 10⁻⁶rad. That of a short distance survey is a few times 10⁻⁵rad. It was confirmed by surveys af- ter construction that a directional accuracy of significantly bet- ter than 1×10⁻⁴rad was attained.

The measured distance between the target and the center po- sition of Super-Kamiokande is 295,335.2±0.7 m. The mea- sured off-axis angle is 2.504±0.004^◦.

3.6.2. Time Synchronization

The T2K GPS time synchronization system builds on expe- rience from K2K, taking advantage of subsequent advances in commercially available clock systems and related technology.

The system provides O(50 ns) scale synchronization between neutrino event trigger timestamps at Super-Kamiokande, and beam spill timestamps logged at J-PARC.

The heart of the system is a custom electronics board called the local time clock (LTC). This board uses a time base de- rived from a commercial rubidium clock, and references it to GPS time using input from two independent commercial GPS receivers. The operational firmware was coded to interface effi-