Introduction to T2K：Clarifying the picture of neutrino mass and mixing

Neutrino and Neutrino Oscillation

Neutrinos are mysterious subatomic particles having a neutral charge. Their existence was first postulated by W.E. Pauli in 1930, to explain the beta-decay of atomic nuclei, and the phenomena was formulated beautifully by E.Fermi (1934). The existence of neutrinos was verified by F. Reines and C. Cowan a quarter of a century later (1956); they conducted an experiment using a reactor as an intense source of neutrinos. It is extremely difficult to detect neutrinos because they can penetrate ordinary matter without any trace or disturbance. Their masses are extraordinarily small –less than 1/1,000,000 of the masses of electrons and the lightest quarks. There are three known types (generations or flavors) of neutrinos –electron neutrinos(ν_e), muon neutrinos(ν_μ), and tau neutrinos(ν_τ)– corresponding to their charged counterparts. Each type of neutrino has a corresponding antiparticle; these are called antineutrinos.

Quarks and leptons are fundamental constituents of matter, which have both three generations. Neutrinos belong to lepton, whose charge is zero. Each of neutrino forms a counterpart with a negatively-charged lepton.

The difference in their extremely small masses causes neutrinos to change flavors during flight. For example, if 100% pure muon neutrinos are generated by an accelerator, they transform into tau neutrinos after covering a certain distance and then revert to their original flavor (muon neutrinos). Hence, this periodic change of neutrino flavors is called neutrino oscillation. The possibility of mixing of neutrino flavors was first proposed by Z. Maki, M. Nakagawa, and S. Sakata (1962).

Discovery of neutrino oscillation and its verification

Neutrino oscillation –a consequence of the finite masses of neutrinos and of the mixing of their flavors– was discovered via Super-Kamiokande collaboration (1998), through the observation of neutrinos produced by primary cosmic rays interacting with the Earth's atmosphere. The zenith-angle distribution of atmoshperic neutrino showed that the number of upward-going muon neutrinos, generated on the other side of the Earth, is half of the number of downward-going ones. Neutrino oscillation causes some of the muon neutrinos changing into tau neutrinos which can not be observed. It was the first experimental indication of minuscule, albeit non-zero, mass differences in neutrino generations.

© Kamioka Observatory, ICRR The University of Tokyo

Zenith angle distribution of the muon neutrinos, generated by primary cosmic rays interacting with the Earth's atmosphere. In the atmosphere, production ratio of the muon neutrino and electron neutrino is roughly 2 to 1. In the case of electron neutrinos, the distribution is right-left(up-down) symmetric, and is well agreed to the expectation without neutrino oscillation (blue line). However, in the right figure, the observed number of upward-going muon neutrinos was half of the predictions. Red line is the theoretical expectation by assuming neutrino oscillation.

 K2K experiment is the first long-baseline neutrino oscillation experiment, connecting 250km between proton synchrotron at KEK (Tsukuba, Ibaraki-Prefecture) and the Super-Kamiokande. During the data taking from 1999 to 2004, 112 accelerator-made neutrino events were collected. The existence of neutrino oscillation was confirmed by the experiment with 99.9985% probability.

The neutrino beam-line constructed at KEK for K2K experiment (left). The energy distribution of the accelerator-made neutrino events observed by Super-Kamiokande (right). The spectrum was distorted, which characterizes the occurence of neutrino oscillation.

Purpose of T2K experiment

According to quantum mechanics, none of the neutrino flavor states has a fixed mass. In fact, each of them appears as a superposition (mixture) of different mass states. The relation can be described completely by using a Maki, Nakagawa, and Sakata (MNS) 3 × 3 mixing matrix that connects the 3 flavor states with the 3 mass states. Among the six independent matrix parameters that can be probed via studies on neutrino oscillation, two remain indeterminate: One is the mixing angle between the first and third generations, denoted by θ₁₃, and the other is a complex phase factor denoted by e^iδ. The latter contributes to the violation of CP (Charge conjugation and Parity) symmetry, which could be responsible for the matter-to-antimatter asymmetry (matter-dominance) in our universe. The effect of the observable amount of CP asymmetry is proportional to sinθ₁₃; thus, the magnitude of θ₁₃ is of great interest to physicists.

The neutrino oscillation between three generations. Primary objective of T2K is to discover νμ→νe oscillation.

A muon neutrino event (left) and an electron neutrino event (right) observed by Super-Kamiokande. The charged muon, produced by the muon neutrino interaction, goes straight in the water. Meanwhile, electron, produced by the electron neutrino interaction, causes electro-magnetic shower. As a result, edge of the ring image is blurred.

The primary objective of T2K is to investigate the last unknown mixing angle θ₁₃ by determining the ν_μ-to-ν_e oscillation –a supplemental mode to known ν_μ-to-ν_τ oscillation– that has not been observed thus far. To achieve this objective, T2K directs high-intensity neutrino beams, produced at a neutrino experimental facility in J-PARC, towards Super-Kamiokande, 295 km west of J-PARC. The intensity of the neutrino beams produced by the neutrino experimental facility is the highest in the world. Super-Kamiokande is located underground –1,000m below a mountain. The neutrinos penetrate iron, concrete shields, and rocks effortlessly and reach Kamioka town merely 0.001s after they are produced. Although most of them continue through the atmosphere into outer space, very small traces are detected in the Super-Kamiokande. The detector is cylindrical in shape. It contains 50,000 tons of purified water, and its inner surface is covered with approximately 11,000 highly sensitive photosensors, each of which is 50cm in diameter. The interaction between neutrinos and nuclei in the water results in the formation of the charged counterparts of the neutrinos, i.e., muons and electrons. The muons and electrons emit weak conical wavefronts along their trajectories and produce ring images on the neighboring photosensors. The neutrino flavors (muon-like or electron-like) and energies can be determined by analyzing these images. The appearance of electron neutrinos denotes ν_μ-to-ν_e oscillation.

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