Precise measurement of the solar neutrino day-night and seasonal variation in Super-Kamiokande-I

The time variation of the elastic scattering rate of solar neutrinos with electrons in Super-Kamiokande-I was fit to the variations expected from active two-neutrino oscillations. The best fit in the large mixing angle solution has a mixing angle of tan²θ=0.55 and a mass squared difference of Δm²=6.3×10^-5eV² between the two neutrino mass eigenstates. The fitted day-night asymmetry of -1.8±1.6(stat)_-1.2+1.3(syst)% has improved statistical precision over previous measurements and agrees well with the expected asymmetry of -2.1%.