Solar neutrino measurements in Super-Kamiokande-IV

Upgraded electronics, improved water system dynamics, better calibration and analysis techniques allowed Super-Kamiokande-IV to clearly observe very low-energy B8 solar neutrino interactions, with recoil electron kinetic energies as low as ∼3.5 MeV. Super-Kamiokande-IV data-taking began in September of 2008; this paper includes data until February 2014, a total livetime of 1664 days. The measured solar neutrino flux is (2.308±0.020(stat)-0.040+0.039(syst))×106/(cm2 sec) assuming no oscillations. The observed recoil electron energy spectrum is consistent with no distortions due to neutrino oscillations. An extended maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the neutrino-electron elastic scattering rate in SK-IV results in a day/night asymmetry of (-3.6±1.6(stat)±0.6(syst))%. The SK-IV solar neutrino data determine the solar mixing angle as sin2θ12=0.327-0.031+0.026, all SK solar data (SK-I, SK-II, SK III and SK-IV) measures this angle to be sin2θ12=0.334-0.023+0.027, the determined mass-squared splitting is Δm212=4.8-0.8+1.5×10-5 eV2.