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Experiment T2K (Tokai to Kamioka)

Updated: 2020-10-07

Introduction

A neutrino is an elementary particle, i.e. the basic component of matter that cannot be divided into smaller elements. Neutrinos belong to the same family of particles as electrons, but unlike them they have no electric charge and are much lighter.

There are three types (called flavours) of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. (muon and tau are the heavier "brothers" of the electron). Oscillation is a quantum-mechanical process in which neutrinos change their flavour as a function of time, i.e. as they move through space.

Each elementary particle has its antiparticle. Antiparticles create antimatter. Consequently, there are also three antineutrinos: electron antineutrino, muon antineutrino, and tau antineutrino.

Antineutrinos also oscillate among themselves, that is, they can change into another type of antineutrinos.

About the project

T2K (Tokai to Kamioka) is an experiment that studies neutrino oscillations and is conducted in Japan by a team of about 500 physicists and engineers from over 60 scientific institutions from a dozen countries in Europe, Asia and North America.

On the east coast of Japan, in Tokai, a beam of muon neutrinos is generated using 30 GeV protons accelerated in the J-PARC (Japan Proton Accelerator Research Complex) facility.

The beam, after 280 meters, passes through a system of two main close detectors of the T2K experiment: one on the beam axis (INGRID), and the other - 2.5° off-axis (ND280). In these detectors, the direction of the beam and the content of different neutrino flavours in the beam as well as the distribution of their energy before the phenomenon of oscillation takes place are examined. Near the place of production the beam is very intense, which allows scientists to study in the near detectors also neutrino interactions.

After traveling another 295 kilometres underground (there is no tunnel there, the beam goes right through the ground!), the neutrino beam passes through the Super-Kamiokande far detector of the T2K experiment located 2.5° off-axis. This detector checks how many of the neutrinos in the beam remained muon neutrinos and how many of them oscillated into electron neutrinos

The average energy of the neutrinos decreases with the deviation from the beam axis. The 2.5° angle was chosen to maximize the probability of oscillations in the far detector, which depends on the ratio of the neutrino energy to the covered distance.

The Super-Kamiokande detector is located in the Mozumi mine, 1000 m below the summit of Mount Ikeno in the Japanese Alps. It is a huge cylinder (40 m high and 40 m in diameter) filled with water. On its walls, there are more than 13,000 photomultipliers, i.e. sensors that record Cherenkov light generated by charged particles produced in neutrino interactions. Cherenkov light is created when a charged particle (e.g. an electron or its heavier brother, a muon) moves faster than light in a medium (in this case in water). Nothing can move faster than the speed of light in vacuum, but in water light moves about 25% slower, so it can be 'overtaken'.

In T2K, instead of a beam of muon neutrinos, a beam of muon antineutrinos can also be produced, for which it is studied how many muon antineutrinos remained in the beam and how many of them converted into electron antineutrinos. By comparing the number of electron neutrinos that appeared in the muon neutrino beam with the number of electron antineutrinos that appeared in the muon antineutrino beam, it is possible to assess whether the probability of oscillations is the same for neutrinos and antineutrinos and thus check whether the symmetry between matter and antimatter is conserved or violated.

The parameter that describes the symmetry violation between matter and antimatter is called the δCPsub> phase and ranges from -180 to 180 degrees. In April 2020 in the Nature journal, the T2K experiment for the first time was able to rule out with a high probability of 99.7% almost half of the possible values ​​for this parameter, and the results so far indicate that breaking this symmetry for neutrinos can be very large. [ T2K Results Restrict Possible Values of Neutrino CP Phase - press release].

The Universe is made of matter and contains very little antimatter (i.e. there is an asymmetry between matter and antimatter). However, in the microworld most of the physical phenomena are symmetrical, that is the same for matter particles and antimatter particles, and the differences observed so far are too small to explain the advantage of matter over antimatter in the Universe. The results provided by the T2K experiment are therefore very important as they show a large difference in neutrino and antineutrino oscillations and thus bring us closer to understanding the mystery of the origin of our matter-dominated Universe.

The T2K experiment has been collecting data since the beginning of 2010. Currently, there is ongoing work on increasing the intensity of the neutrino beam, upgrading the ND280 near detector, and in a longer term on constructing a 5 times larger Hyper-Kamiokande far detector and the intermediate water Cherenkov detector (approximately 1-2 km from the beam production place). This is to ultimately confirm whether the matter/antimatter symmetry is broken in neutrino oscillations, and if so, to what extent.

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IFJ contribution

Engineers from DAI (Division of Scientific Equipment and Infrastructure Construction ) of IFJ PAN were involved in the construction of the ND280 detector, which included the design and construction of an assembly system for the SMRD (Side Muon Range Detector) sub-detector.

Physicists from Department 15 of IFJ PAN are involved in the study of neutrino interactions in the ND280 near detector. A good knowledge of their interactions (including the probability of different types of interaction, types and energies of particles produced by these interactions, etc.) is essential for precise measurements of oscillation parameters as well as for other neutrino-related research.

The engineers from DAI have again got involved in the current works on the ND280 detector upgrade, within which they design and build components for new time projection chambers [DAI contribution to the T2K experiment (2008-2009)] the SMRD (Side Muon Range Detector) sub-detector.

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Main results

The main results obtained by the T2K experiment include:

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