Light for Detecting Neutrinos and Gravitational Waves

Abstract

More than 30 years ago, Kamiokande experiment began in the underground of Kamioka, Japan. It was a large detector with 3000 ton of water and 1000 photomultiplier tubes. It was designed to observe proton decays. However, proton decays were not observed. Therefore, Prof. Masatoshi Koshiba proposed to modify the detector to observe solar neutrinos. In fact, after the detector improvement, Kamiokande observed neutrinos from a supernova explosion and solar neutrinos. Because of the importance of the studies of these neutrinos, Super-Kamiokande was constructed. Super-Kamiokande is a very large neutrino detector with 50,000 ton of water and 13,000 photomultiplier tubes. The experiment began in 1996. In 1998, neutrino oscillation was discovered by the studies of atmospheric neutrinos. Further studies on neutrinos are in progress in Kamioka. Gravitational waves are generated by the violent motion of heavy mass. Therefore, gravitational waves are expected to be a very important new tool to observe the Universe such as the mergers of blackhole binaries or those of neutron star binaries. In fact, gravitational waves have been observed in the LIGO and Virgo laser interferometers. KAGRA is a project trying to observe gravitational waves with 3 km $\times 3$ km arm lengths laser interferometer. It has been constructed in the underground of Kamioka, Japan. KAGRA has unique features such as the underground site to minimize the effect of seismic noises and the cryogenic mirrors to reduce the thermal noises.