A NEW TREND OF COSMIC-RAY PHYSICS IN JAPAN - NEUTRINO AND UNIVERSE -
YOJI TOTSUKA
A new detector, Super-Kamiokande, with 50,000 ton water and 11,200 large-size photomultipliers, was built and is now operational to study neutrinos coming from the Sun or neutrinos produced in EarthÕs atmosphere. The size of the detector is unprecedented in the field of cosmic-ray physics and will set a standard for the future cosmic-ray experiments. The observation has been very smooth and the data corresponding to about 400-day observation have already been analyzed.
I would like to report in my talk some interesting results from Super-Kamiokande:
1. The sun generates energy by nuclear processes in its core. Neutrinos are inevitably produced during these processes. We observed only 40% of neutrino events compared to the standard solar model prediction. This results confirmed the previous experiments with an order of magnitude more events. The question is then which is the case, either that the Sun is not properly understood and producing less energy at the solar core now, or that the Sun is working as the theory says but neutrinos change themselves along the way from the solar core to the detector on Earth.
2. Cosmic rays entering the atmosphere interact with oxygen and nitrogen nuclei and produce mesons which eventually decay and produce neutrinos of the mu-type and the e-type. The ratio of mu-type to e-type should be 2 at some appropriate energy range based on the simple decay schemes. Super-Kamiokande observed the ratio 1.2 in stead of 2. Moreover the upward going neutrinos have even stronger deficit. The neutrinos must have changed their identify during traversal in Earth.
3. The above observations strongly suggest that so called neutrino oscillations take place, which change one type of neutrinos to another type. In order for the oscillations to take place neutrinos must have masses. The finite neutrino masses force us to rewrite some part of the particle theory, and it is hence very important.
4. If neutrinos do have masses, they will affect very much the fate of the Universe. The neutrino masses might determine whether the Universe will expand forever or the Universe will eventually halt the expansion and recollapse again.