T2K experiment

Long baseline neutrino experiment in Japan
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.

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.

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. 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.

Three Time Projection Chambers (TPCs) are gas tight rectangular boxes, with a cathode plane in the centre and readout MicroMegas modules at both sides parallel to the cathode. TPCs are filled with argon based drift gas under atmospheric pressure. Charged particles crossing TPC ionise 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 MicroMegas modules, and X coordinate is based on the electrons drift time. In the magnetic field, the curvature of this path allows to determine charge and momentum of the particle, and the amount of the ionisation electrons per unit distance is used to identify particles based on the Bethe Bloch formula.

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 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.

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 providing additional information relevant for their identification.

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 stopped in the calorimeter. SMRD can also act as a trigger for cosmic rays. Finally, it can help identify beam interactions in the surrounding walls and in the magnet itself.

WAGASCI BabyMIND is a new detector located next to the INGRID and ND280 detectors, devoted to neutrino interaction studies. It provided the first neutrino beam data using a full detector setup during the 2019/2020 winter run.

T2K is a successor of the KEK to Kamioka (K2K) experiment, which ran from 1999 till 2004. In the K2K experiment, an accelerator beam of muon neutrinos was produced at KEK facility in Tsukuba (Japan) and sent towards the Super Kamiokande detector, located 250 km away. The K2K experiment results confirmed at the confidence level of 99.9985% (4.3 σ) the disappearance of the muon neutrinos and were consistent with the previous measurements of oscillation parameters measured by the Super Kamiokande detector for atmospheric neutrinos. 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 installed in the fall of 2010. T2K far detector is the large Super Kamiokande detector, which has been running since 1996 and studying proton lifetime and oscillations of atmospheric, solar and accelerator neutrinos. with 55% of data in neutrino mode and 45% in antineutrino mode. The building of an additional Intermediate Water Cherenkov detector at a distance of around is also considered for the HK experiment. In February 2020, the proton beam power reached 515 kW with 2.7x1014 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 2.0x1014 protons per pulse, while for 1.3 MW the repetition cycle has to be further decreased to 1.16 s and the number of protons per pulse has to increase to 3.2x1014. In addition to increasing the primary proton beam power, the current in the horns focusing secondary particles (pions, kaons, etc.) with a chosen electric charge 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 the anti neutrino mode beam) by around 5   10%. Reduction of the repetition cycle will require a series of hardware upgrades, including a major upgrade of the Main Ring power supplies 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 cooling capacity of the secondary beamline components such as the graphite target, the magnetic horns and the beam dump, as well as disposal of a larger amount of irradiated cooling water. The installation of the new sub detectors into ND280 will be done in 2022.

The SuperFGD is a detector consisting of approximately 2 million 1 cm3 scintillating polystyrene cubes. The cubes are woven with a series of optical fibres 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 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 antineutrino energy. The latter consists of applying a layer of resistive 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. 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. Cherenkov energy threshold (minimal total energy of a charged particle to produce Cherenkov light) 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 absorbed by another nucleus, which goes into an excited state and during deexcitation produced gamma rays. High energy photons (for gadolinium their total energy is about 8 MeV) scatter electrons from an atom and/or produce electron positron pairs, which then produce Cherenkov light. Gadolinium is a naturally occurring element with the highest cross section on the capture of neutrons at thermal energy. For 25 meV neutrons, the cross section for gadolinium is about 105 times higher than for hydrogen. 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. The first loading of 13 tons of Gd2(SO4)3·8 (gadolinium(III) sulfate octahydrate) 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 remote supernova neutrinos, for which 's are the most reactive in SK but were yet indistinguishable from neutrinos from other sources. It will also improve the detector performance for supernova explosions in our galaxy and study better matter antimatter differences in accelerator neutrino oscillations.