What is Solar Neutrino?
The energy source of the Sun is the nuclear fusion in the core of the Sun. A Helium atom and electron neutrinos are generated from 4 protons. These emitted neutrinos are called solar neutrinos.
This figure shows the sun observed
by Super-Kamiokande, which can measure
the direction of the neutrinos.
It takes about a hundred thousand years that the gamma ray generated by the nuclear fusion in the center of the Sun appears on the surface. On the other hand, solar neutrinos arrive at the earth in about eight minutes because neutrinos rarely interact with the matter. Therefore we see the solar activity a hundred thousand years ago by the visible lights, however, it is possible to see the inner of the Sun in real-time by neutrinos.
Solar Neutrino Problem
The detection of solar neutrinos has started by the Homestake experiment (R. Davis) in 1968. They observed the solar neutrinos using the interaction with chlorine turned to Argon. As a result, only about 30% of the expected number of solar neutrinos by the solar model was observed. After that, Kamiokande experiment and other experiments started the observation of solar neutrinos and they also observed 30-50% of the expectation. The physicists could not solve this solar neutrino problem for about 30 years.
Solar Neutrino Oscillation
Super-Kamiokande presented the results of the precise measurement of the solar neutrino flux and the spectrum in June 2000. The results confirm about 45% deficits of solar neutrinos at the confidence level more than 99.9% and indicate the solar neutrino problem can be explained by the neutrino oscillation. Furthermore the precise measurement of the spectrum and day-night variation of the solar neutrino constrained the neutrino oscillation parameters and strongly suggested large mixing.
In June 2001, the data from Super-Kamiokande combined with the data from SNO experiment revealed strong evidence of solar neutrino oscillations without referring to the solar model flux calculations and also confirmed the precision of the flux calculation by the solar model.