Neutrino facility and neutrino physics in J-PARC

........................................................................................................................................................ The neutrino facility in the Japan Proton Accelerator Research Complex (J-PARC) produces a very intense neutrino beam for the Tokai to Kamioka long-baseline neutrino oscillation experiment, T2K. The physics operation of the T2K started in January 2010 and the data taken from the January 2010 to March 2011 period correspond to 1.43 × 10 protons on target. From analysis of these data, the first nm ne appearance candidates were observed and an indication of sin2u13 = 0 was obtained with 2.5s significance. The T2K aims for a very precise measurement of ne appearance and a possible search for CP violation with very large statistics. ........................................................................................................................................................


Introduction
The neutrino is a fundamental particle that only interacts very weakly with mattervia weak interaction.The neutrino was assumed to be massless for a long time.After the discovery of neutrino oscillation by Super Kamiokande (SK) in 1998 [1], studies on this phenomenon have progressed very far in the last fifteen years.Since neutrino oscillation is allowed in cases where the masses of neutrinos are not zero (in fact, differences in mass between different types of neutrinos are not zero), it is concluded that the neutrino masses are finite but very small.In the neutrino oscillation literature, weak eigenstates (n e , n m , n t ) and mass eigenstates (n 1 , n 2 , n 3 ) are expressed with a unitary 3 × 3 matrix (Maki -Nakagawa-Sakata (MNS) matrix) using three mixing angles and a CP violating phase as [2] where S ij (C ij ) stands for sinu ji (cosu ij ) and d is a complex phase.Measurements of the mixing angle u 12 have been performed by several experiments by detecting neutrinos from the sun (solar neutrinos) and anti-neutrinos from reactors (reactor neutrinos) [3][4][5].Measurements of the angle u 23 are taken by detecting neutrinos produced in the atmosphere (atmospheric neutrinos) and acceleratorproduced neutrinos (accelerator neutrinos) [6][7][8].Measurements of the angle u 13 are in progress now by detecting accelerator neutrinos and reactor neutrinos [9 -13].Measurements of the mixing angles are listed in Table 1.u 13 appears in Eq. 1 in combination with the unknown complex phase d.Therefore, the measurement of the angle u 13 is very important in neutrino studies.The Tokai to Kamioka long-baseline neutrino oscillation experiment (T2K) [14]. is an acceleratorbased neutrino oscillation experiment.The neutrino facility at the Japan Proton Accelerator Research Complex (J-PARC) [15][16][17].was designed and constructed for the T2K experiment.A very intense neutrino beam produced at J-PARC is detected by a far neutrino detector, SK, located 295 km away from J-PARC (Fig. 1).The SK is a 50 kton water Cherenkov detector, which is a well-understood neutrino detector that has operated for 16 years, since 1996.The physics motivations of the T2K experiment are mainly covered by two topics: 1) determination of the angle u 13 with an undiscovered n m n e oscillation (n e appearance), and 2) precise measurement of the angle u 23 and mass difference Dm 23 2 with n m n m oscillation (n m disappearance).T2K aims to measure the angle u 13 with 20 times better sensitivity than the current measurement, and the angle u 23 and the mass difference Dm 23 2 with precisions of 1% and 3%, respectively.
The T2K neutrino beamline has two important features: 1) high intensity and 2) off-axis nature.The intensity of the T2K neutrino beam is about 100 times larger than the former KEK to Kamioka long-baseline neutrino oscillation experiment (K2K).Such an intense neutrino beam is derived from J-PARC's very intense proton accelerator.The designed beam power of the J-PARC proton synchrotron is 750 kW; 3.3 × 10 14 protons accelerated to 30 GeV and extracted to the neutrino beamline once in a 2.1 sec repetition cycle.Secondly, this is the first utilization of an off-axis neutrino beam [18]; the direction to SK is pointed with an angle of 2-38 with respect to the proton beam axis.Figure 2(a) shows a plot of neutrino energy as a function of parent pion momentum for
One is an increase in neutrino flux around the 0.6 GeV region compared to the on-axis (off-axis angle equals zero) case.Another is the great suppression of high energy neutrinos (.1.5GeV) that cause background events produced through neutral current interactions.This paper presents the features of the J-PARC neutrino facility in Sect.2, the operational status and physics results in Sect.3, and future prospects in Sect. 4.

Neutrino facility
The J-PARC neutrino facility is composed of three sections: a primary proton beamline, a secondary beamline, and a near neutrino detector complex.Each component is described in details in this section.

Primary beamline
The purposes of the primary beamline are 1) to direct the proton beam toward the SK, and 2) to focus the proton beam with appropriate position, direction, and size on the target.The primary beamline is composed of two normal-conducting (NC) sections, i.e., a preparation section and a final focus section, and one super-conducting (SC) section between them, as shown in Fig. 3.All the magnets are aligned to the designed location with a precision of about 0.2 mm.

2.
1.1 Super-conducting section.The proton beam is extracted from the main ring (MR) to the north direction, while the SK is located to the west of J-PARC.Therefore, the extracted beam is bent by about 808 in order to be directed toward SK.This is achieved by use of the strong magnetic field of super-conducting magnets.
The SC section consists of 28 combined-function magnets, the world's first super-conducting combined-function magnets, periodically distributed along the 150 m-long tunnel.Two successive magnets with opposite gradients form a focusing -defocusing doublet and are put in a cryostat and cooled by super-critical helium.The magnet is capable of producing a 2.6-T-dipole field and a 19-T/m-quadrupole field at a current of 7345 A to transport a 50 GeV proton beam.At present it is operated at 4350 A for a 30 GeV beam.All the magnet coils are connected in series, with the same current.Three sets of super-conducting steering coils are therefore installed at appropriate locations to tune the beam orbit.
The SC section has a large acceptance of 190 p.This is much bigger than the 81p of the NC section, and enables minimization of the beam loss in the super-conducting magnets.
2.1.2Normal-conducting section.The proton beam extracted from the MR first comes into one NC section, called the preparation section.The purpose of this section is to adjust the beam optics parameters to fit the SC section.
One of the most important features of the preparation section magnets is its high radiation tolerance.The most-upstream four magnets out of 11 are wholly made from mineral material to withstand the high radiation dose due to beam loss.Quick couplers are used for cooling-water and electricity PTEP 2012, 02B005 T. Sekiguchi 3/18 connectors to minimize work time near the beamline.These magnets were designed and constructed by a magnet group of the hadron facility and have the same design.
The proton beam is bent to the SK direction via the SC section and comes into the final focus section.The purpose of the final focus section is targeting: it focuses the proton beam onto the graphite target with appropriate position, direction, and size.Angle control is essential to generate a wellcontrolled off-axis beam.

Beam monitors.
In order to minimize beam loss, it is essential to measure the beam position and sizes precisely.For this purpose, 21 beam position monitors, 19 profile monitors, 5 intensity monitors, and 50 beam loss monitors are distributed along the beamline.
The beam position monitors (ESMs) are of electro-static type equipped with four electrodes, top/ bottom/left/right, surrounding the beam orbit cylindrically.They measure the asymmetry of the beam-induced charge of the opposite-side electrodes.A position resolution of slightly better than 0.5 mm is achieved.
The beam profile monitors (SSEMs) use secondary emission electrons from segmented titanium foil generated by proton beam bombardment.Vertical (horizontal) strips of 5-mm-thick titanium foils are arranged in plane to measure the horizontal (vertical) beam profile by measuring the charge from each strip.Beam size and position are measured with an accuracy of 0.2 mm.Since the foil causes beam loss of about 10 25 , the profile monitors are inserted into the beamline only during beam tuning.They are extracted from the beamline during usual data taking, and the ESMs are used for beam position monitoring.In addition to the SSEM, another beam profile monitor, an optical transition radiation monitor (OTR), is installed just in front of the target.The OTR has thin titanium-alloy foils, placed at 458 to the proton beam.The emitted light, reflected and transported by four aluminum parabolic mirrors, is detected by a camera, located in a low radiation area outside radiation shielding blocks, to produce an image of the proton beam profile.Unlike the SSEM, the foil is placed in front of the target during usual data taking and monitors the beam profile all the time.
The beam intensity monitors (CTs) are ferromagnetic ring cores with 50-turn coils wound on them.The induced current waveform by the beam passage through the ring core is measured with 160 MHz FADC.The beam current and timing is measured with 2% and 3 ns accuracy, respectively.
The beam loss monitors (BLMs) are installed at almost every magnet and measure the secondary particles generated by beam losses.They provide the most sensitive information on the amount of beam loss and the current beam condition.Though it is difficult to isolate the origin of the loss, we can get some general loss distribution along the beamline.The BLM is made of a single-wire proportional counter filled with Ar-CO 2 gas.If its signal is above a certain threshold, a beam abort signal is generated.Using the beam loss generated by the SSEM foil, the BLMs are measured to be sensitive to a beam loss of about 16 mW.
2.1.4Vacuum system.In order to keep a good vacuum level of 10 26 Pa, eight vacuum pumps, six ion pumps, and two turbo-molecular pumps are distributed along the NC section.This vacuum level is required to connect the beamline to the MR as well as to reduce the heat load to the SC section, which realizes a vacuum of 10 28 Pa owing to the liquid-helium temperature of the beam tube.Fast-closing valves are installed at the boundaries to the MR and the SC to protect their good vacuum even in the case of vacuum failure in the NC section.
2.1.5Beam operation stability.Stability of the beam direction, the most important parameter in order to target SK, is continuously monitored by muon monitors to be better than 0.1 mrad, corresponding to 30 m at SK.This is much better than the required stability of 1 mrad.

Secondary beamline
Protons delivered through the primary beamline hit the production target.Produced secondary particles, mainly pions, are focused into the decay volume and decay in flight to muon neutrinos and muons inside the decay volume.All the hadrons, as well as muons below 5 GeV/c, are absorbed by a beam dump at the end of the decay volume.Muons above 5 GeV/c can penetrate the beam dump and are monitored to characterize the neutrino beam.
The secondary beamline consists of the target station, the decay volume, the beam dump, and muon monitors (Fig. 4).Beamline components including the target, magnetic horns and beam dump are contained in an iron vessel whose volume is 1500 m 3 .The iron vessel is filled with helium gas to reduce pion absorption and to suppress tritium and NO x production by the beam.The helium gas is circulated and temperature-controlled through a heat exchanger.Water cooling channels called plate coils are welded to all the surfaces of the helium vessel and 30 8C water cools the vessel to prevent its thermal deformation.

Target station.
The target station is a facility building for the target, magnetic horns, and the helium vessel, equipped with a remote handling system for the instruments inside.The upstream part A maintenance area with manipulators, cameras, a moving stage, and a lead-glass window is set up for maintenance of highly radioactive beamline equipment.

Target.
The target is a highly purified graphite rod with a density of 1.8 g/cm 3 .It is 91.4 cm long (1.9 interaction length) and has a 2.6 cm diameter.The graphite rod is surrounded by a 2 mm thick graphite tube and the whole graphite assembly is contained in a titanium case (Fig. 5).The thickness of the titanium case is 0.3 mm to reduce interaction between the particles and heat deposit.The target assembly is installed on the upstream of the first horn and the target rod itself is placed inside the first horn inner conductor.The relative positioning accuracy between the target and the first horn axis is 0.1 mm.The target graphite rod is cooled by helium gas flowing through the gaps between the core and the outer tube and between the tube and the case.The maximum temperature reaches 700 8C with 750 kW proton beam exposure.

Magnetic horns.
Three magnetic horns are located downstream of the target for focusing.With the horns, the neutrino flux at SK is increased by a factor of 16 at the spectrum peak energy of 0.6 GeV.The horns have a structure of two coaxial (inner and outer) conductors (Fig. 6).The conductors are made of aluminum alloy (6061-T6) with a tensile strength of 310 MPa.The aluminum alloy is stronger than copper but the conductivity is about 43% of copper.A toroidal field proportional to 1/r, where r is the distance from the horn axis, is produced in a volume between the inner and outer conductors.The maximum field when the horns are operated with a rated current of 320 kA is 2.1 T. The dimensions of the horns are listed in Table 2.The thickness of the inner conductor is 3 mm to reduce pion absorption.The horn conductors are cooled by water.Since aluminum is very sensitive to acid and alkaline, the circulation water is kept around neutral pH.Hydrogen produced by water dissociation due to beam exposure is recombined with oxygen by a hydrogen recombiner.
A pulse power supply produces a 32 kA, 5 ms wide pulsed current.The current is fed to a transformer with a turn ratio of 10, and the output current of the transformer is increased up to 320 kA.In the secondary circuit of the transformer, three horns are connected in series with four parallel aluminum striplines.The total stored energy in the 7.5 mF capacitor bank inside the power supply is 375 kJ.40% of the total energy is consumed due to Joule loss and the rest of the energy is returned  to the capacitor bank, which is reused for the next pulse.The repetition rate of the operation cycle is currently 0.4 Hz.The current in each stripline is monitored by Rogowski coils, whose intrinsic measurement accuracy is 1%.The outputs of the coils are read out by 200 kHz FADCs and the pulse shape information is recorded.The estimated uncertainty for the current measurement is less than 2%.

Decay volume.
The decay volume is a 96-m-long tunnel with steel walls, with a gradually increasing cross section of 1.4 m width and 1.7 m height at the upstream end, and 3.0 m width and 5.0 m height at the downstream end.Pions generated at the target decay in-flight into muons and muon neutrinos while passing through this tunnel.A significant fraction of generated particles and their decay products generate a huge amount of heat and radiation.The decay volume is therefore surrounded by 6 m thick reinforced concrete for radiation shielding, and 40 plate coils are welded on the steel wall to keep the temperature below 100 8C.

Beam dump.
The beam dump is located in the most downstream of the decay volume, 109 m from the target.The core of the beam dump is made of graphite (1.7 g/cm 3 ), and is located in the helium vessel to avoid oxidization.Its size is 3.2 m long, 1.9 m wide, and 4.7 m high, with a total weight of 50 tons.Aluminum cooling plates are attached to the graphite core to keep the temperature at the center of the core around 150 8C with a 750 kW beam.A number of thermocouple wires are distributed in the graphite core to monitor the temperature and its distribution inside the core.

Muon monitor.
The muon monitor is located just behind the beam dump and outside the helium vessel, 118 m from the target.Muons from pion decay above 5 GeV/c pass through the beam dump and are measured.The direction and yield of muons are measured to monitor the neutrino beam direction and intensity.The precision for the beam direction measurement is better than 0.25 mrad, which corresponds to a 3 cm precision of the muon profile center.The precision of monitoring the stability of the neutrino beam intensity is better than 3%.The muon monitor is composed of two independent detectors: ionization chambers and silicon PIN photodiodes.Each detector has 7 × 7 sensors placed at 25 cm intervals and covers a 150 × 150 cm 2 area in total.The two-dimensional muon profile reconstructed from the distribution of the observed charge is independently measured in each detector.

Near neutrino detector
On-axis and off-axis detectors called ND280 (Fig. 7) are built in the neutrino monitor pit located 280 m downstream of the production target [14].The on-axis detector is set on the proton beam axis to measure the neutrino beam profile.The off-axis detector measures the muon neutrino flux, energy spectrum, and intrinsic electron neutrino contamination in the beam in the direction of SK, along with measuring rates for exclusive neutrino reactions.

On-axis detector (INGRID).
The purpose of the on-axis detector (INGRID) is to provide daily measurements of the neutrino beam profile and intensity.The beam center is measured to a precision better than 10 cm, corresponding to 0.4 mrad precision in the beam direction.INGRID consists of 14 identical modules arranged in a cross shape, 7 modules horizontal and 7 vertical.Each INGRID module consists of a sandwich structure of iron plates and tracking scintillator planes surrounded by veto scintillator planes to reject charged particles produced by interactions outside the module.The total iron mass serving as a neutrino target is 7.1 tons per module.

Off-axis detector.
Off-axis detectors are installed in the magnet formerly used in the UA1 experiment at CERN.It provides a 0.2 T horizontal dipole magnetic field with 2900 A for measurement of charged particle momentum.The dimensions of the magnet are 7.0 m × 3.5 m × 3.6 m (inner volume) and 7.6 m × 5.6 m × 6.1 m (external), and the weight of the yoke is 850 tons.Inside the magnet there are several detectors installed as shown in Fig. 8: a pi-zero detector (PØD), three time-projection chambers (TPCs), two fine-grained detectors (FGDs), and a downstream electromagnetic calorimeter (ECal).The PØD, TPCs, and FGDs are all surrounded by electromagnetic calorimeters (barrel and downstream ECal).The return yoke of the magnet is instrumented with a side-muon range detector (SMRD).The PØD, FGDs, ECal and SMRD are plastic scintillator detectors with WLS and MPPC readout.The PØD is to detect p 0 for the measurement of the neutral current process on a water target.The PØD consists of four groups.The two ECal groups are sandwiches of 7 scintillator modules alternating with stainless-steel-clad lead sheets (4 mm thick).The two water target groups are sandwiches of scintillator modules alternating with 28 mm thick water bag layers and 1.5 mm thick brass sheets.The PØD operates with either full or empty water target bags, which enables determination of the water target cross section with a subtraction method.
The three TPCs [19] measure the momentum of charged particles produced by neutrino interactions in the detector, and measure dE/dx for particle identification.Each TPC consists of an inner box that holds an argon-based drift gas contained within an outer box that holds CO 2 as an insulating gas.Ionization electrons produced in the TPC drift away from the central cathode and toward the readout plane of micromegas [20].There are 72 micromegas modules with 7.0 mm × 9.8 mm anode pad segmentation.The obtained point spatial resolution is typically 0.7 mm per column of pads and the obtained dE/dx resolution is 7.8% for minimum ionizing particles with the lowest 70% truncated mean, which is better than the requirement of 10%.
The FGDs provide the target mass for neutrino interactions and also measure the direction and ranges of recoil protons produced by charged-current (CC) interactions in the FGDs, giving clean identification of CC quasi-elastic (QE) and CC non-QE interactions.The two FGD modules (2300 mm (W) × 2400 mm (H) × 365 mm (D)), placed after the first and second TPCs, consist of layers of finely segmented scintillating tracker bars (9.61 mm × 9.61 mm × 1864.3 mm).One FGD module consists entirely of plastic scintillator, while the second consists of plastic scintillator and water to allow separate determination of exclusive neutrino cross sections on carbon and on water.
The ECal is a segmented Pb-scintillator detector surrounding the PØD, TPCs, and FGDs mainly to measure gamma-rays that do not convert in the inner detectors.It is critical for the reconstruction of p 0 decays.The ECal is made of 13 independent modules of three different types: 6 barrel modules surrounding trackers, 6 PØD modules surrounding the PØD, and one downstream module.
Air gaps in the magnet return yoke are instrumented with plastic scintillator detectors, SMRD, to measure the ranges of muons that exit from inner detectors.The SMRD also provides a veto for events entering the detector from the outside and a trigger for ND280 calibration.3. Operational status and physics results

Operational status
The first beam commissioning of the neutrino beamline was held in April 2009.On April 23rd, the first detection of muons produced with neutrinos was done by the muon monitors.All the beamline components, except for the second and third magnetic horns and some of the near neutrino detectors, functioned successfully.3.
The neutrino beam condition during RUN I and RUN II was very stable.The proton beam center, angle, and width at the target were within +0.4 mm, +0.1 mrad, and 4.0 + 0.2 mm, respectively, for the whole run periods.The targeting efficiency was 99.4 + 0.6%.The neutrino beam direction and intensity were measured by INGRID.The event rate in INGRID (1.5 events/10 14 p.o.t.) was stable and consistent with expectations.The neutrino beam direction was within +0.3 mrad for the whole run period, which satisfied the +1 mrad requirement (Fig. 10).A total of 2474 419 spills were retained for analysis after beam and far detector quality cuts, yielding 1.43 × 10 20 p.o.t.

Physics results
Physics results based on the data taken in RUN I and II periods are presented in this section.The analysis procedure for n e appearance and n m disappearance studies is basically composed of prediction of neutrino beam fluxes at SK and neutrino event selection in SK.The neutrino beam flux prediction is performed by Monte Carlo (MC) simulation.The hadron production model in MC is tuned with experimental data from NA61 measurements [21].The predicted neutrino fluxes at SK are shown in Fig. 11.The estimated uncertainties of the n m and n e fluxes below 1 GeV are around 14%.The main sources of the flux uncertainties come from hadron production uncertainty.Proton beam uncertainty, such as uncertainty in the alignment of secondary beamline components, and beam direction uncertainty also contribute to the flux uncertainty, but they are small (at most 2%).To predict the number of neutrino events in SK, cross sections of neutrino interactions are necessary.MC simulation for neutrino interaction is primarily performed by a NEUT event generator [22] and cross-checked by a GENIE generator [23].The MC generator is tuned with recent neutrino interaction data [24 -26].A n m charged-current (CC) measurement in the off-axis near detector is used to constrain the expected event rate at SK. Figure 12 shows the MC simulation is in good agreement with data.
At SK, neutrino events with charged-current quasi-elastic (CCQE) interactions are used for n e appearance and n m disappearance studies.A neutrino event selection in SK starts from selecting a fully-contained fiducial volume (FCFV) sample.The SK volume is divided into two regions, inner detector and outer detector (the outermost 2-m-thick region).The fiducial volume of 22.5 kton is defined by further eliminating the 2-m-thick region from the surface of the inner detector.The reconstructed vertices for neutrino interactions are required to be within the fiducial volume, there should be no activity in the outer detector, and beam-synchronized timing is also required.

n e appearance study.
For n e CCQE event selection, a single e-like ring is required.With further requirements of no delayed signal (to separate n m events), a p 0 rejection cut (to eliminate NC events), electron energy .100 MeV, and reconstructed neutrino energy , 1250 MeV, 6 events are retained as n e candidates.The reconstructed neutrino energy spectrum of these events is shown in Fig. 13.The predictions for the number of neutrino events are 1.5 + 0.3 (5.5 + 1.0) for sin 2 2u 13 ¼ 0 (0.1).The total systematic uncertainty is +22.8 −22.7 % +17.6 −17.5 % for sin 2 2u 13 ¼ 0 (0.1), where the main contributions come from the uncertainties of neutrino flux, cross section, and far detector (Table 4).The probability of observing six or more candidate events under the hypothesis of sin 2 2u 13 ¼ 0 is 7 × 10 23 , or sin 2 2u 13 ¼ 0 is excluded with 2.5s significance.The 68% and 90% confidence level (C.L.) regions for sin 2 2u 13 are calculated using the method of Feldman and Cousins [27] (Fig. 14).The confidence intervals for sin 2 2u 13 are 0.03(0.04), sin 2 2u 13 , 0.  and to determine the angle u 13 with better statistics.This result was published in June 2011 [9].After our publication, the MINOS and Double CHOOZ Collaborations also published their results, indicating sin 2 2u 13 = 0 with smaller significances [10,11].The Daya Bay and RENO Collaborations recently published their results with higher significances, sin 2 2u 13 ¼ 0.092 + 0.017 (Daya Bay) and sin 2 2u 13 ¼ 0.113 + 0.023 (RENO), respectively [12,13].Both results confirmed the T2K results with higher significances, and so sin 2 2u 13 = 0 is established.| are calculated using the method of Feldman and Cousins (Fig. 16).An alternate analysis with a maximum likelihood method is performed for comparison.The best-fit value from this .The alternating analysis is consistent with that from the Feldman and Cousins method.This result is published in Ref. [28].The result of n m disappearance is consistent with those from MINOS and SK [8,29].

Future prospects
For precise measurements of both n e appearance and n m disappearance, we need to enlarge the statistics with higher beam intensity.The approved T2K run condition is 15 000 h operation with 750kW beam power, which corresponds to about 8 × 10   Once a tighter relation between sin 2 2u 13 and d CP is obtained from n e appearance measurement, combination with sin 2 2u 13 from reactor experiments can constrain the value of the CP phase (Fig. 17).This implies that the comparison between reactor and accelerator measurements gives a hint for CP violation in leptons.
In terms of the n m disappearance study, precise measurements of sin Beyond the scope of the T2K, n e appearance measurements with anti-neutrinos to search for CP violation in leptons are very important for the neutrino experiment facility.A comparison of the difference in n e appearances between neutrinos and anti-neutrinos is a direct investigation of CP violation.For much higher statistics, a multi-MW proton accelerator and a huge neutrino detector with new technology should cooperate for this study.

Conclusion
The features of the neutrino facility in J-PARC and physics derived from the facility are described in this paper.Successful operation of the T2K neutrino beamline has been ongoing since January 2010 and data corresponding to 2.56 × 10 20 p.o.t.have been accumulated as of May 15 th 2012.From the analysis using data taken in the RUN I and II periods, corresponding to 1.43 × 10 20 p.o.t., the n e appearance study obtains an indication of n e appearance with 2.5s significance.The first result of the n m disappearance study is derived from the data and is consistent with those from other experiments.T2K aims to establish sin 2 2u 13 with 5s significance by accumulating 1 × 10 21 p.o.t. by summer 2013.Finally, possibilities for future physics outputs with large statistics are discussed.
vessel is located in the underground area of the target station.The helium vessel, made of 10 cm thick steel, is 15 m long, 4 m wide, and 11 m high.The top lid of the vessel is removable for replacement of the target and magnetic horns.Two-meter-thick iron shielding blocks and 1-m-thick concrete shielding blocks are placed above the target and magnetic horns inside the vessel, and 4.5-mthick concrete shielding blocks are installed above the helium vessel.Once all these shielding blocks are removed, all the equipment inside the helium vessel can be handled by a numerically-controlled overhead crane and remote-controlled hoisting attachments in case of maintenance and replacement.

Fig. 4 .
Fig. 4. A schematic figure of the secondary beamline in side view.

Fig. 5 .
Fig. 5.A photograph of the target to be installed to the first magnetic horn.

Fig. 6 .
Fig. 6.A schematic figure of the first magnetic horn.

Fig. 7 .
Fig. 7. Near detector ND280 at the neutrino monitoring site.The off-axis detector is located on the upper level, horizontal INGRID modules are located on the level below, and the vertical INGRID modules span the bottom two levels.

Fig. 8 .
Fig. 8.An exploded view of the ND280 off-axis detector.(One half of the magnet yoke is not shown.) The second and third magnetic horns and the rest of the near neutrino detectors were installed in summer and fall 2009.Beam commissioning of all the components installed was done in December 2009.Operation for physics data taking was started in January 2010.The whole data taking period from January 2010 to June 2012 was divided into three run periods, RUN I (from January 2010 to June 2010), RUN II (from November 2010 to March 2011), and RUN III (from March 2012 to June 2012).Beam power from MR has been gradually increasing since January 2010; the proton beam intensity in units of the number of protons per pulse and the accumulated number of protons on target (p.o.t.) are shown in Fig. 9. Typical beam powers for RUN I, II, and III are 60, 135, and 190 kW, respectively.The proton beam conditions in these run periods are listed in Table

Fig. 9 .
Fig. 9.The accumulated number of protons on target (shown by the solid line) and the number of protons on target per pulse (shown as dots) as of May 15th 2012.

Fig. 10 .Fig. 11 .
Fig. 10.Stability of the neutrino beam profile center in horizontal (x) and vertical (y) directions for the RUN I and II periods, measured by INGRID.Errors are only statistical.The dashed lines correspond to the +1 mrad requirement for the neutrino beam direction.

Fig. 12 .
Fig. 12. Measured neutrino energy spectrum of n m CC events reconstructed in the off-axis near neutrino detector.The data are shown using points with error bars (statistical only) and the MC predictions are shown as histograms.

Fig. 14 .
Fig. 14.The 68% and 90% C.L. regions for sin 2 2u 13 for each value of d CP derived from the observed number of events in the three-flavor oscillation case for normal (upper) and inverted (lower) mass hierarchies.The systematic uncertainties are taken into account.The best fit values are shown with solid lines.

Fig. 15 .
Fig. 15.Reconstructed neutrino energy spectrum of the 31 data events (dots) compared with the expected spectra in SK without oscillation (dashed line) and with best-fit oscillations (solid line).
21 p.o.t. at 30 GeV.A near-term milestone by summer 2012 is to double the statistics compared to data taken in the RUN I and II periods (i.e.1.43 × 10 20 p.o.t.) in order to establish sin 2 2u 13 = 0 with more than 3s significance by n e appearance.The expectation for the accumulated number of p.o.t. by the end of RUN III is 3.2 × 10 20 p.o.t. at maximum.The next near-term milestone by summer 2013 is to achieve more than 5s significance with 1 × 10 21 p.o.t.To achieve this milestone, 300 kW beam power is necessary.Possible physics outputs with large statistics such as 8 × 10 21 p.o.t. are as follows.The first priority is very precise measurement of sin 2 2u 13 with n e appearance.Once sin 2 2u 13 is measured with

Fig. 17 .
Fig. 17.Hypothetical 68% C.L. region for sin 2 2u 13 as a function of CP phase d CP , expected from the approved p.o.t. of T2K, 3.75 MW .10 7 s (¼8 × 10 21 p.o.t.).The vertical axis for sin 2 2u 13 is shown in arbitrary units.The hypothetical sin 2 2u 13 region from reactor measurements is also shown in the hatched region.

Table I .
Summary of measurements of neutrino mixing angles.

Table 2 .
Dimensions of the magnetic horns.

Table 3 .
Proton beam conditions in the RUN I, RUN II and RUN III periods.The accumulated number of protons on target (p.o.t.) for RUN III is based on the data taken as of May 15 th 2012.

Table 4 .
13ntributionsFig.13.Reconstructed neutrino energy spectrum of the events that pass all n e appearance signal selection criteria except for the energy cut.The vertical line shows the applied cut at 1250 MeV.Histograms show the predicted neutrino energy spectrum assuming sin 2 2u 13 ¼ 0.1.