Solar neutrino analysis of Super-Kamiokande

Super-Kamiokande-IV data taking began in September of 2008, and with upgraded electronics and improvements to water system dynamics, calibration and analysis techniques, a clear solar neutrino signal could be extracted at recoil electron kinetic energies as low as 3.5 MeV. The SK-IV extracted solar neutrino flux between 3.5 and 19.5 MeV is found to be (2.36$\pm$0.02(stat.)$\pm$0.04(syst.))$\times {10}^6$ /(cm${}^2$sec). The SK combined recoil electron energy spectrum favors distortions predicted by standard neutrino flavour oscillation parameters over a flat suppression at 1$\sigma$ level. A maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the elastic neutrino-electron scattering rate in SK, results in a day/night asymmetry of $-3.2\pm 1.1$(stat.)$\pm$0.5(syst.)$$. The 2.7 $\sigma$ significance of non-zero asymmetry is the first indication of the regeneration of electron type solar neutrinos as they travel through Earth’s matter. A fit to all solar neutrino data and KamLAND yields ${\sin }^2{\theta }_{12}=0.304\pm 0.013$, ${\sin }^2{\theta }_{13}=0.031_{-0.015}^{+0.017}$ and $\Delta m^2_{21} = 7.45^{+0.20}_{-0.19} \times 10^{-5} {\rm eV}^2$.