Neutrino Experiments

Study of neutrino oscillation with accelerator neutrino beam: T2K experiment

T2K is a long baseline neutrino experiment using the J-PARC accelerator complex and Super-Kamiokande, 295 km away. By combining both muon-type and electron-type neutrino interaction events observed at the Super- Kamiokande detector, we placed the world best constraint on the neutrino mixing angle θ23, and the first-ever constraint on the CP asymmetry parameter in the lepton sector.

T2K has released the first neutrino oscillation results from anti-neutrino data in 2015. A discussion is underway towards a possible extension of T2K data taking (called T2K Phase II) with an upgrade of near detectors. Such an extension will allow us to observe CP violation in the lepton sector with a significance of 3σ, if the parameters are in a favorable region.

In order to improve the systematic uncertainty from neutrino-nucleus interaction cross sections, we have proposed a new experiment at J-PARC neutrino beam facility. The experiment, named WAGASCI, was approved as a test experiment at J-PARC. We have constructed a prototype of WAGASCI detector, and installed it in front of existing T2K near detector to measure the neutrino cross section on water.

Search for proton decays: Super-Kamiokande

Proton decay is the only way to directly prove the Grand Unified Theory, which is an attractive candidate for a model of physics beyond the Standard Model. We aim to enhance the sensitivity to proton decay at Super-Kamiokande with an improved event reconstruction.

Next generation large water Cherenkov detector: Hyper-Kamiokande project

In order to pursue the study of neutrino properties beyond T2K, we propose the next generation water Cherenkov detector, Hyper-Kamiokande (Hyper-K). One of the main goals of Hyper-K is the search for CP violation in the leptonic sector using accelerator neutrino and anti-neutrino beams. The sensitivity to the CP violating phase is studied with full simulation by our group. It is shown that with Hyper-K and J-PARC accelerator, CP violation can be observed after five years of experiment for a large part of possible parameter space. The sensitivity to proton decay lifetime, which is expected to be an order of magnitude better than current Super-K sensitivity, is also studied in our group.

R&D of new generation photodetectors

For next generation experiments, we have been pursuing R&D of new generation photodetectors with our expertise in semiconductor detectors. We have been involved in the development of MPPC from its infancy, and continuously working on the improvement of its performance.

As a candidate of photosensor for Hyper-K, we have been developing hybrid photodetector (HPD) combining a large-format phototube technology and avalanche diode as the photo-electron multiplier.