T2K ("Tokai to Kamioka") is a particle physics experiment studying the oscillations of the accelerator neutrinos. The experiment is conducted in Japan 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 CERN experiment (RE13). T2K collected data within its first phase of operation from 2010 till 2021. The second phase of data taking (T2K-II) is expected to start in 2023 and last until commencement of the successor of T2K – the Hyper-Kamiokande experiment in 2027.
T2K was the first experiment which observed the appearance of electron neutrinos in a muon neutrino beam. It also provided the world best measurement of oscillation parameter θ23 and a hint of a significant matter-antimatter asymmetry in neutrino oscillations. The measurement of the neutrino-antineutrino oscillation asymmetry may bring us closer to the explanation of the existence of our matter-dominated Universe.
The intense beam of muon neutrinos is produced in the J-PARC facility (Japan Proton Accelerator Research Complex) in Tokai on the east coast of Japan. The beam is directed towards the Super-Kamiokande far detector located away in the city of Hida, Gifu prefecture. The properties and composition of the neutrino flux are first measured by a system of near detectors located 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.
Physics program
T2K experiment was proposed in 2003 with the following measurement goals: The discovery of the → oscillations, and thus the confirmation that the last unknown mixing angle θ13 is not zero. Precise measurement of the oscillation parameters Δm and θ23 via muon neutrino disappearance studies. Search for sterile neutrino oscillations, which could be observed as a deficit of neutral current neutrino interactions. Measurements of various interaction cross-sections for different types of neutrinos and targets in an energy range of few GeV.
Since the start of the data taking in 2010, the T2K experiment succeeded to provide a list of world-class results: The confirmation of electron neutrino appearance in the muon neutrino beam (→), which was the first time when neutrinos produced in one flavour was explicitly observed in another flavour. The most precise measurement of the θ23 parameter. Limits on a sterile neutrino oscillation parameters based on studies in the near ND280 and far Super-Kamiokande detectors. Various cross-section measurements of electron and muon neutrinos and antineutrinos, including inclusive charged current (CC) interactions, CC interactions without pions and with single pion in the final state, coherent pion production, neutral current interactions, etc. on different targets such as carbon, water and iron. The first significant constraint on the δCP parameter, responsible for the matter-antimatter asymmetry in the neutrino oscillations.
δCP takes values from -π to π (i.e. from −180° to 180°) and can be measured by comparing oscillations of neutrinos to those of antineutrinos. The CP symmetry would be conserved, and thus the oscillation probabilities would be the same for neutrinos and antineutrinos, for δCP equal to 0 or ±π. T2K provided the first and the strongest yet constraint on δCP, 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 conditions proposed by the Russian physicist Andrei Sakharov, necessary to produce the excess of matter with respect to antimatter at the early universe, which forms now our matter-built Universe. CP violation 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 leptogenesis and thus such measurement would be important step to understand how the Universe were formed.
The NOvA experiment is the other neutrino oscillation experiment capable to measure δCP through the comparison between → and → 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 Fermilab and far detector in Ash River, Minnesota. NOvA provided a less precise measurement of δCP, 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.
Future upgrades of T2K is expected to provide more precise measurements of Δm and θ23 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 δCP phase and confirmation if the CP symmetry is conserved or violated in the neutrino oscillation at the 3σ significance level in the T2K-II and 5σ in the Hyper-Kamiokande experiment.
Neutrino beam
T2K uses a muon neutrino or muon antineutrino beam produced at the J-PARC facility using a proton beam gradually accelerated to 30 GeV by a system of three accelerators: 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 synchrotron (Main Ring). Protons collide with a graphite target, producing mesons, mainly pions and kaons, which are then focused by a set of three magnetic horns 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 and , forming a muon neutrino beam, while negative pions and kaons decay mainly into and , forming a muon antineutrino beam. All remaining hadrons and charged leptons 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.
Off-axis beam
T2K is the first experiment in which the concept of off-axis neutrino beam was realized. The neutrino beam at J-PARC is designed so that it can be directed 2 to 3 degrees away from the Super-Kamiokande 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 is maximal for around 600 MeV neutrinos. In this neutrino energy range, the dominant type of neutrino interactions are charged current 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 with meson production, which are background in the oscillation analysis in the T2K experiment.
Near detectors
The near detector complex is located at a distance of 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: INGRID detector (Interactive Neutrino GRID) located on the axis of the neutrino beam, ND280 detector located 2.5° away from the beam axis, i.e. at the same angle as the far detector. 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 energy.
Signal readout
Except for the Time Projection Chambers in ND280, the entire active material (enabling particle tracking) of the near detectors is plastic scintillator. The light produced by traversing charged particles in the plastic scintillator bars and planes is collected by wavelength-shifting fibres and detected by Hamamatsu Multi-pixel photon counters 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.
INGRID detector
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 modules outside the cross. The height and width of the arms are . A single module consists of alternating layers of iron and a plastic scintillator. An additional 4 vetoVeto 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 detector. 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.
ND280 detector
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 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.
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 UA1 experiment producing 0.2 T uniform horizontal magnetic field and instrumented with scintillator planes constituting the Side Muon Range Detector.
Pi-Zero detector
The Pi-Zero () 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.