Abstract
The development of long baseline neutrino oscillation experiments with accelerators in Japan is reviewed. Accelerator experiments can optimize experimental conditions, such as the beam and the detector, as needed. The evidence of neutrino oscillations observed in atmospheric neutrinos was confirmed by the K2K experiment. The T2K experiment discovered the oscillation channel from a muon neutrino to an electron neutrino with the squared mass difference varDelta m^2 sim 2.5 times 10^{-3}~mathrm{eV}^2, and presents a full analysis with all three flavor states of both neutrinos and anti-neutrinos. The three mixing angles, which quantify the mixing of the three neutrino states, have been measured. They are large compared to those of quarks, especially the mixing of the second and third generation which is nearly maximal. The T2K experiment has an ongoing program producing new results, and recently has shown a hint of CP violation in neutrinos. Future generation long baseline neutrino experiments beyond T2K are expected to clarify the relationship between flavor and mass, as well as measure possible CP violation in neutrino oscillations.
Highlights
1.1 Historical overviewThe question of whether neutrinos are massless or massive has fascinated physicists for many years, for several reasons
The observation is consistent with the oscillation hypothesis. – the point with the highest likelihood is outside of the physical region, = (1.2, 2.6 × 10−3eV 2), the probability that sin2 2θ ≥ 1.2 if the true parameters are at the best fit physical parameters is 26.2%, based on Monte Carlo (MC) virtual experiments. – The probability that the observations are due to a statistical fluctuation instead of neutrino oscillation is estimated by computing the log-likelihood ratio of the null oscillation case to the best fit point
In the initial data set of 1.43 × 1020 POT accumulated by March 2011 corresponding to 6.5% of the present data, the first time evidence of non-zero θ13 was reported [57] by observing νμ → νe appearance
Summary
The question of whether neutrinos are massless or massive has fascinated physicists for many years, for several reasons. Because of the short oscillation length (∼ km scale for the mass square difference of eV2 with neutrino energy of GeV), studies near the eV scale require high rejection power of νμ to look for small components of oscillation production in a νμ beam, since the wrong flavor components (other than νμ) in an accelerator neutrino beam are already known to be small. One of the critical measurements is to see that νμ from an accelerator shows the deficiency under similar conditions This requires about 100 km baseline with a GeV neutrino beam, looking for the disappearance of νμ. Yoji Totsuka was leading the studies of neutrinos at SuperKamiokande with an accelerator beam, in addition to the cosmic neutrinos He was the central figure of the K2K and T2K experiments from the very beginning as spokesperson of Super-Kamiokande and later as Director General of KEK. This was the starting point of the neutrino oscillation experiments with accelerator beams in Japan
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