T2K experiment - Knowledge

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modules outside the cross. The height and width of the arms are 10 metres (33 ft). A single module consists of alternating layers of iron and a plastic scintillator. An additional 4 veto layers of the scintillator surround the module on the sides to distinguish particles entering from the outside from those produced by interactions inside the module. The total mass of iron in one module is 7.1 tons and constitutes 96% of the module weight. On the neutrino beam axis, in the middle of the cross between the vertical and horizontal arm, there is an additional module built only from layers of the plastic scintillator (Proton Module) with a mass of 0.55 tons. Its purpose is to register quasielastic interactions and compare the obtained results with the simulations.

2013:. The fraction of neutrons that will be captured in SK is 50% for 0.01% Gd concentration and 90% for 0.1% concentration – the planned final Gd concentration in SK. The signal from neutron capture is delayed by a fraction of a millisecond (the time the neutron travels across the water before the capture plus the time when Gd remains in the excited state) with respect to the charged lepton signal and usually appears within a distance of 50 cm (the distance travelled by the neutron before the capture) from the neutrino interaction point. Such a double flash event (the first flash from the charged lepton, the second flash from the Gd deexcitation photons) is a signature of an antineutrino interaction. 1713:

additional final state particles: high angle particles thanks to the increased angular acceptance, and less energetic particles because of lower detection thresholds. This detector acceptance improvement is important to cover almost the same phase space available at the far detector (SK). In addition, final state particles will allow probing nuclear effects which are essential for constraining the systematic effects of the oscillation analysis. It is an important step as well in the transition to using semi-inclusive or exclusive models in neutrino oscillation physics, as opposed to current inclusive models which use only the final state lepton in their predictions.

881:) Detector (P0D) contains 40 plastic scintillator module planes, which in the central part are interleaved with 2.8 cm thick bags fillable of water and thick brass sheets, and in two peripheral regions scintillator modules are sandwiched with lead sheets. By comparison of the amount of interaction between modes with and without water in the bags, it is possible to extract the number of neutrino interactions occurring on the water – the target material inside the far detector Super-Kamiokande. The size of the entire active P0D volume is around 2.1 m × 2.2 m × 2.4 m (X×Y×Z) and its mass with and without water is 15.8 and 12.9 tons respectively. 106:. The properties and composition of the neutrino flux are first measured by a system of near detectors located 280 metres (920 ft) from the beam production place at the J-PARC site, and then again in the Super-Kamiokande detector. Comparison of the content of different neutrino flavours in these two locations allows measurement of the oscillations probability on the way between near and far detectors. Super-Kamiokande is able to detect interactions of both, muon and electron neutrinos, and thus measure the disappearance of muon neutrino flux, as well as electron neutrino appearance in the beam. 1182: 1627:

maintain their sandwiched structure and continue to detect forward going leptons and high momentum hadrons. The upstream part which now hosts the P0D sub-detector will be replaced by three novel sub-detectors: a scintillating 3D target (Super Fine-Grained Detector or SuperFGD), two new TPCs on top and below the SuperFGD (High-Angle TPCs or HATPCs), and six Time-of-Flight (TOF) detectors surrounding the new structure. Each of these sub-detectors is briefly described below. The installation of the new sub-detectors into ND280 will be done in 2022.

551: 1054: 565: 539: 856: 822: 810: 1564: 757: 1587:, as well as too high momentum threshold to reconstruct a large part of produced pions and knocked-out nucleons (protons and neutrons). In Charged Current Quasi-Elastic (CCQE) interactions, the dominating interaction in the ND280 near detector, kinematics of produced lepton is enough in the reconstruction of the incoming neutrino energy. However, other types of neutrino interactions in which additional particles ( 1663: 1019:

scintillator layers only, while the second FGD is composed of alternating layers of scintillator and water. The second FGD is partially composed of water because the Super-Kamiokande detector is water-based. Cross sections on carbon and on the water can be determined from a comparison of neutrino interactions in the two FGDs.

1985:) is proportional to the particle mass, and in water it equals 0.8 MeV for electrons, 160 MeV for muons and 1400 MeV for protons. Thus, protons released in neutrino interactions often fall below the threshold and remain undetected. Neutron, as a neutral particle, does not produce Cherenkov light. However, it can be 1274:, respectively. Due to relatively large mass, muons usually do not change their direction and thus produce a well-defined cone of Cherenkov light observed by PMTs as a clear, sharp ring. In contrast, electrons, because of smaller mass, are more susceptible to scattering and almost always produce electromagnetic 1626:

The Upgrade of the ND280 detector (ND280 Upgrade) addresses these requirements by replacing a part of the P0D sub-detector with three types of new sub-detectors. The existing downstream part, consisting of two Fine-Grained scintillation Detectors (FGDs) and three Time Projection Chambers (TPCs), will

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In February 2020, the proton beam power reached 515 kW with 2.7x10 protons per pulse and with 2.48 seconds between pulses (so-called repetition cycle). To reach 750 kW, the repetition cycle will be reduced to 1.32 s with 2.0x10 protons per pulse, while for 1.3 MW the repetition cycle has to

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The construction of the neutrino beamline started in 2004 and it was successfully commissioned in 2009. Construction of the entire INGRID detector and majority of the ND280 detector (without barrel part of the electromagnetic calorimeter) was completed in 2009. The missing part of the calorimeter was

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designed to detect the light emitted by the particles produced during the interactions in the target. Unlike the current FGDs, the SuperFGD has a three-fold projective 2D readouts providing a quasi-3D readout. This readout configuration increases the detection of short tracks almost uniformly in all

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will also be increased from 250 kA to 320 kA. This will increase the amount of right-sign neutrinos (neutrinos in the neutrino mode beam and anti-neutrinos in the anti-neutrino mode beam) by 10%, and reduce the amount of wrong-sign neutrinos (anti-neutrinos in the neutrino-mode beam and neutrinos in

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The new detector will provide measurements of various charged-current neutrino interaction channels with high precision, lower momentum threshold and full angular acceptance. These will constrain flux and cross-section models uncertainties for the particles produced at high angles. These assets will

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The Electromagnetic Calorimeter (ECal) surrounds the inner detectors (P0D, TPCs, FGDs) and consists of scintillator layers sandwiched with lead absorber sheets. Its role is to detect neutral particles, especially photons, and measure their energy and direction, as well as to detect charged particles

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The main purpose of the INGRID detector is the monitoring of the direction and intensity of the beam on a daily basis by direct detection of neutrino interactions. The INGRID detector consists of 16 identical modules arranged in the shape of a cross, 7 in a vertical and 7 in a horizontal arm, plus 2

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far detector and one of the near detectors, ND280. The average energy of neutrinos decreases with the deviation from the beam axis. The off-axis angle was chosen to 2.5° to maximize the probability of oscillation at a distance corresponding to the far detector, which for 295 kilometres (183 mi)

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modules like a sandwich. The modules can be rearranged easily to adapt the magnetic field to the particular needs of the experiment. The magnetic field is created only inside the ferrite so it is very power efficient compared to magnets that have to magnetize empty spaces around them like the ND280

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The Side Muon Range Detector (SMRD) consists of scintillator modules which are inserted into the gaps in the magnet. The SMRD records muons escaping the inner parts of the detector at large angles with respect to the beam direction. The remaining types of particles (except for neutrinos) are mostly

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The ND280 detector is used to measure the flux, energy spectrum and electron neutrino beam pollution for the same off-axis angle as for the far detector. ND280 also investigates various types of muon and electron neutrino and antineutrino interactions. All this allows estimating the expected number

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the gas along their track. The ionisation electrons drift from the cathode to the sides of the TPC, where they are detected by the MicroMegas providing a 3D image of a path of the traversing charged particle. Y and Z coordinates are based on the position of the detected ionisation electrons on the

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quasielastic interactions, for which it is possible to reconstruct the energy of the interacting neutrino only on the basis of the momentum and direction of the produced charged lepton. The higher neutrino energies are suppressed by the off-axis configuration, decreasing the number of interactions

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The impact the ND280 Upgrade will have on the analyses at T2K is two-fold. Firstly, an increase in statistics thanks to the 2 ton SuperFGD target will allow to nearly double the amount of data in certain samples. Secondly and more relevant, the new configuration will allow for better detection of

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ND280 is composed of the set of inner sub-detectors: Pi-Zero detector and a tracker with 2 Fine-Grained Detectors interleaved with 3 Time Projection Chambers, placed inside of a metal frame called a basket. The basket is surrounded by the electromagnetic calorimeter and a magnet recycled from the

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loaded into the Super-Kamiokande far detector was taken. In 2021–2022 a major upgrade of the neutrino beamline and the ND280 near detector will be performed. From 2023 till 2026 neutrino data will be taken within the second phase of the T2K experiment (T2K-II). In 2027, the successor of the T2K

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Two Fine-Grained Detectors (FGDs) are placed after the first and second TPCs. Together the FGDs and TPCs make up the tracker of ND280. The FGDs provide the active target mass for the neutrino interactions and are able to measure the short tracks of proton recoil. The first FGD is composed of

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Location at the same distance of 280 meters from the graphite target as ND280 and INGRID detectors, but at a different off-axis angle of 1.5 degrees, causes that the energy spectrum of the neutrino beam is peaked around different energies for each of the off-axis angles corresponding to the

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also facilitate detection of low momentum hadrons produced in the interaction of the neutrino with bounded states of 2 nucleons or through reinteractions inside the target nucleus of particles produced by the neutrino, and thus better modelling of such interactions in the far detector.

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in March 2011. The proton beam power, and thus the neutrino beam intensity, was constantly growing, reaching by February 2020 the power of 515 kW and a total number of accumulated protons on target of 3.6×10 protons with 55% of data in neutrino-mode and 45% in antineutrino-mode.

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WAGASCI-BabyMIND (WAter Grid SCIntillator Detector – prototype Magnetized Iron Neutrino Detector) is a magnetised neutrino detector located at 1.5° off-axis angle, built to explore the energy spectrum variation with the off-axis angle and cross-sections at higher average neutrino

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and a minor upgrade of the focusing horn power supplies, all of which will be installed during the long shutdown in 2021. Increasing the horn current will require using an additional (third) horn power supply. Meanwhile, the higher proton beam power demands enhancement of the

346:, rejecting at the 3σ (99.7%) significance level almost half of the possible values, ruling out the both CP conserving points at the significance level of 95% and giving a strong hint that CP violation may be large in the neutrino sector. The CP violation is one of the 488:, which is in slight tension with the T2K result. The T2K best-fit point lies in the region disfavoured by NOvA at the confidence level of 90%. There are ongoing works to obtain a joint fit to data from both experiments to quantify consistency between them. 735:

The near detector complex is located at a distance of 280 metres (920 ft) from the graphite target. Its purpose is to measure the neutrino flux before oscillations and to study neutrino interactions. The system consists of three main detectors:

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for each crossing track with a timing resolution of the order of 140 ps. The capability to determine track direction sense has been proven in the actual ND280 to be critical to reduce background generated outside the active inner detectors.

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Finally, the total fiducial volume (the mass available for neutrino interactions) of the tracker part of the ND280 detector, characterised with a better reconstruction ability, needs to be enlarged in order to increase the rate of neutrino

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Veto is a part of a detector where no activity should be registered to accept an event. Such requirement allows constraining the number of background events in a selected sample; here the background from particles produced outside of the

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will be built. Part of the beam related upgrade works and the upgrade of the ND280 detector will be performed yet before the start of phase II of the T2K experiment. The HK experiment is expected to start operation around the year 2027.

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T2K experiment started to take neutrino data for a physics analysis in January 2010, initially with an incomplete ND280 detector, and starting from November 2010 with the full setup. The data taking was interrupted for one year by the

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in quark section was confirmed already in 1964, but it is too small to explain the observed matter-antimatter imbalance. The strong CP violation in the neutrino sector could lead to matter excess production through the process called

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One BabyMIND (prototype Magnetized Iron Neutrino Detector) that is a magnetized muon spectrometer to detect forward-going muons. BabyMIND sports an original configuration of scintillation modules intertwined with magnetized

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decay can be mis-reconstructed as an electron in the Super-Kamiokande detector, thus this reaction can mimic electron neutrino interactions and constitute an important background in electron neutrino appearance measurement.

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located at one or both ends of the fibres. Scintillator bars are organised into layers, where bars in two neighbouring layers are perpendicular to each other providing together 3D information about the traversing particles.

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High-angle and backwards-going tracks must be well-reconstructed. This is achieved by increasing the angular acceptance and the efficiency of the discrimination between backward from forward going tracks using timing

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modules technology for track reconstruction. The main novel feature of the HATPCs, aside from their high angle coverage, is the use of the resistive MicroMegas technology. The latter consists of applying a layer of

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material to increase the charge-sharing capabilities of the MicroMegas modules. This reduces the number of readout channels and allows for a spatial resolution which is as good as the one in the current TPCs.

2045:) into SK water was done in July–August 2020 and lead to a 0.011% concentration of Gd. T2K collected its first data with Gd in SK in March–April 2021. Usage of gadolinium-doped water will allow studying 1434:

Phase II of the T2K experiment is expected to start at the beginning of 2023 and last until 2026, following by the commencement of the HK experiment. The physics goals of T2K-II are a measurement of the

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detectors (WAGASCI, WAter-Grid-SCIntillator-Detector) that act as the main water targets and particle trackers. The 3D grid-like structure of scintillator bars creates hollow cavities filled with water.

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parameters, cross-section measurements which will extend our understanding of neutrino interactions and thus improve theoretical models used in neutrino generators, as well as further constrain on the

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O cross-section estimate from the CH one. The fraction of water in WAGASCI water-scintillator modules is 80% enabling a measurement of the charged-current neutrino cross-section ratio between water (H

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by the international cooperation of about 500 physicists and engineers with over 60 research institutions from several countries from Europe, Asia and North America and it is a recognized

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T2K Collaboration (September 2019). "Measurement of the muon neutrino charged-current cross sections on water, hydrocarbon and iron, and their ratios, with the T2K on-axis detectors".

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T2K Collaboration (21 June 2016). "Measurement of double-differential muon neutrino charged-current interactions on C8H8 without pions in the final state using the T2K off-axis beam".

1583:), but it also has a number of limitations, like low reconstruction efficiency of particles produced almost perpendicular and backward with respect to the direction of the interacting 2100:(HK) experiment, will use the upgraded system of the currently used accelerator and neutrino beamline and upgraded set of the near detector. Apart from that, a new far detector, the 469:

oscillation channels. NOvA is conducted in the United States and measures accelerator neutrino oscillation at the distance of 810 km on the way between beam production place in

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directions. Due to its geometry and coupled with the TOF and the HATPCs, the SuperFGD has the capability to detect fast-neutrons, which could be useful in the reconstruction of the

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and directed into a tunnel called the decay volume. Depending on the horns polarity, either positive or negative particles are focused. Positive pions and kaons decay mainly into

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T2K Collaboration (21 February 2020). "First combined measurement of the muon neutrino and antineutrino charged-current cross section without pions in the final state at T2K".

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Hyper-Kamiokande Proto-Collaboration (19 May 2015). "Physics potential of a long-baseline neutrino oscillation experiment using a J-PARC neutrino beam and Hyper-Kamiokande".

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Hyper-Kamiokande Proto-Collaboration (19 May 2015). "Physics Potential of a Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino Beam and Hyper-Kamiokande".

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The SuperFGD is a 2 by 2 by 0.5 metres (6 ft 7 in × 6 ft 7 in × 1 ft 8 in) detector consisting of approximately 2 million 1 cm

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and type of interactions in the far detector, reducing the systematic error in the neutrino oscillations analysis associated with models of neutrino interactions and flux.

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detector. The building of an additional Intermediate Water Cherenkov detector at a distance of around 2 kilometres (1.2 mi) is also considered for the HK experiment.

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be further decreased to 1.16 s and the number of protons per pulse has to increase to 3.2x10. In addition to increasing the primary proton beam power, the current in the

1674:(HATPCs) will surround the SuperFGD in the plane perpendicular to the incoming neutrino beam. Their design is similar to that of the existing TPCs, as they both use the 4574: 3367:

T2K Collaboration (26 January 2017). "First measurement of the muon neutrino charged current single pion production cross section on water with the T2K near detector".

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T2K Collaboration (27 February 2020). "Measurement of the charged-current electron (Anti-)neutrino inclusive cross-sections at the T2K off-axis near detector ND280".

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T2K Collaboration (31 October 2014). "Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation gamma rays".

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T2K Collaboration (2015). "Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with 6.6E20 protons on target".

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in neutrino oscillations. The measurement of the neutrino-antineutrino oscillation asymmetry may bring us closer to the explanation of the existence of our

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one. However, the magnetic field is not homogeneous over the travel volume of the muons, and this poses a still open challenge for momentum reconstruction.

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The detector needs to efficiently detect the nucleons in the final state of neutrino interactions. For this, the detection thresholds need to be lowered.

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T2K Collaboration (7 May 2013). "Measurement of the inclusive numu charged current cross section on carbon in the near detector of the T2K experiment".

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T2K Collaboration (11 December 2015). "Measurement of the numu charged-current quasielastic cross section on carbon with the ND280 detector at T2K".

589:: first to 400 MeV energy by the Linac linear accelerator, then up to 3 GeV by the RCS (Rapid Cycle Synchrotron), and finally up to 30 GeV by the MR 6642: 2070:'s are the most reactive in SK but were yet indistinguishable from neutrinos from other sources. It will also improve the detector performance for 5130: 6099: 4844: 4799: 4703: 4520: 4314: 4247: 4188: 3964: 3876: 3621: 3562: 3495: 3420: 3353: 3286: 3219: 3152: 3085: 3020: 2953: 2886: 2819: 2752: 2484: 2425: 2347: 998:

MicroMegas modules, and X coordinate is based on the electrons drift time. In the magnetic field, the curvature of this path allows to determine

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are stopped by a 75-ton block of graphite (so-called beam dump) and in the ground, while neutrinos travel underground towards the far detector.

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of the registered particle. That means it is not possible to distinguish neutrino from antineutrino interaction based on a charge of produced

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T2K Collaboration (30 April 2019). "Search for light sterile neutrinos with the T2K far detector Super-Kamiokande at a baseline of 295 km".

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T2K Collaboration (2014). "Precise Measurement of the Neutrino Mixing Parameter θ23 from Muon Neutrino Disappearance in an Off-Axis Beam".

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in the reconstructed neutrino energy spectrum. Thus, it is essential to optimize the detector to be sensitive to additional particles and

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Super-Kamiokande detector is located 1000 m underground in the Mozumi Mine, under Mount Ikeno in the Kamioka area of Hida city. It is a

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installed in the fall of 2010. T2K far detector is the large Super-Kamiokande detector, which has been running since 1996 and studying

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Two WallMRD (Wall Muon Range Detector) that are non-magnetized muon spectrometers to detect side going muons. They are made of passive

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of measurements from these detectors will provide an improved constraint on the neutrino cross-sections as a function of their energy.

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T2K Collaboration and J-PARC Neutrino Facility Group (14 August 2019). "J-PARC Neutrino Beamline Upgrade Technical Design Report".

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phase and confirmation if the CP symmetry is conserved or violated in the neutrino oscillation at the 3σ significance level in the

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T2K Collaboration (2011). "Indication of Electron Neutrino Appearance from an Accelerator-produced Off-axis Muon Neutrino Beam".

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experiment (RE13). T2K collected data within its first phase of operation from 2010 till 2021. The second phase of data taking (

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