Operation of the Super-Kamiokande (SK) II detector in Japan was terminated last October after three years of running to begin a full restoration of the detector. Precise studies on neutrinos will resume next June.
The SK detector consists of a cylindrical tank containing 50,000 tonnes of pure water viewed by about 11,000 photomultipliers (PMTs) of 50 cm diameter. The water tank is 40 m in height and 40 m in diameter, and located 1000 m underground. Neutrinos interact with the water and give rise to Cherenkov light, which provides information about the neutrino energy, direction and type or flavour. In 1998, the collaboration announced that neutrinos change flavour – oscillate – which is possible only if the particles have mass. The evidence came from observing neutrinos created by cosmic-ray interactions in the atmosphere. This was followed in 2001 by evidence for the oscillations of solar neutrinos in the combined data from SK and the Sudbury Neutrino Observatory. More recently, the KEK-to-Kamioka (K2K) experiment, using a man-made neutrino beam from KEK to the SK detector has confirmed the oscillations observed in the atmospheric neutrinos.
Several thousands of PMTs in the detector were destroyed in November 2001, when the shock wave from the implosion of one PMT at the bottom of the tank triggered a chain reaction of implosions in more than half the PMTs (see CERN Courier January/February 2002 p6). In 2002, the detector was partially reconstructed using about 5000 PMTs encased in plastic covers to avoid a similar accident. This partial reconstruction was done quickly in only a year in order to continue the K2K experiment. After three years of operation as SK-II, with half the original density of PMTs, the long awaited full reconstruction of the detector has now begun. Next June, the detector’s third phase, SK-III, will start to take data again.
The discovery of neutrino oscillations has opened up a new window of research with a variety of subjects for SK to tackle. An experiment using an intense neutrino beam from Tokai – Tokai-to-Kamioka (T2K) – is expected to start in 2009. The beam will be produced by a 50 GeV proton synchrotron being constructed at the Japan Proton Accelerator Research Complex in Tokai (see CERN Courier November 2004 p41). SK-III will be the far detector at a distance of 295 km from the beam-production point. The T2K experiment will determine neutrino oscillation parameters precisely and search for effects of the neutrino mixing angle, θ13, which is so far unobserved.
A longer exposure to atmospheric neutrinos will be important in searching for a resonant matter effect in the Earth and may help to resolve the octant ambiguity in the mixing angle θ23. At the lower energies of solar neutrinos, an up-turn in the spectrum is expected as direct evidence for large-mixing-angle solutions and will provide precise oscillation parameters. The higher statistics from several years of exposure should allow this measurement.
SK could also detect several thousand neutrino interactions from a galactic supernova. Such a large number of events would reveal details of the supernova explosion mechanism, as well as information on the properties of neutrinos. The positive identification of electron-antineutrinos in SK could also be possible in future. Neutrons emitted in antineutrino interactions could be detected through the 2.2 MeV gamma-rays emitted by neutron capture on protons and through interactions with gadolinium dissolved in the pure water.
Lastly, the detection of nucleon decay as predicted by grand unified theories has always been one of the primary topics for SK. Sensitivity to the decay mode p →e+ + p0 will soon reach the level corresponding to a lifetime of 1034 years. Decay modes favoured by supersymmetry, which include K mesons in the final state, will become interesting with a longer exposure in SK-III, and the collaboration hopes to observe the first indication of nucleon decay in the near future.
