Solar neutrino analysis of Super-Kamiokande

Abstract

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 $-3.2\pm1.1$(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 $\sin^2 \theta_{12} = 0.304 \pm 0.013$, $\sin^2 \theta_{13} = 0.031^{+0.017}_{-0.015}$ and $\Delta m^2_{21} = 7.45^{+0.20}_{-0.19} \times 10^{-5} {\rm eV}^2$.