The Solar Neutrino Day/Night Effect in Super-Kamiokande

The time variation of the elastic scattering rate of solar neutrinos with electrons in Super-Kamiokande-I was fit to the day/night variations expected from active two-neutrino oscillations in the Large Mixing Angle region. Combining Super-Kamiokande measurements with other solar and reactor neutrino data, the mixing angle is determined as sin θ = 0.276 −0.026 and the mass squared difference between the two neutrino mass eigenstates as ∆m = 7.1 −0.5×10 eV. For the best fit parameters, a day/night asymmetry of −1.7±1.6(stat) −1.2(syst)% was determined from the Super-Kamiokande data, which has improved statistical precision over previous measurements and is in excellent agreement with the expected value of −1.6%.