Solar Neutrino Results from Super-Kamiokande

TL;DR

This paper reports on solar neutrino measurements from Super-Kamiokande, including flux, energy spectrum distortions, and neutrino oscillation parameters, with improved detection at low energies and analysis of day-night asymmetry.

Contribution

The study presents new solar neutrino flux measurements and oscillation parameters using upgraded Super-Kamiokande data combined with previous phases, enhancing understanding of neutrino flavor change.

Findings

Solar neutrino flux: (2.37 ± 0.015 (stat) ± 0.04 (syst)) × 10^6/(cm^2 sec)

Day-night asymmetry: -3.2 ± 1.1 (stat) ± 0.5 (syst) %

Oscillation parameters: sin^2θ_12=0.305±0.013, Δm^2_21=7.49^{+0.19}_{-0.17}×10^{-5} eV^2

Abstract

Super-Kamiokande-IV (SK-IV) data taking began in September of 2008, after upgrading the electronics and data acquisition system. Due to these upgrades and improvements to water system dynamics, calibration and analysis techniques, a solar neutrino signal could be extracted at recoil electron kinetic energies as low as 3.5 MeV. When the SK-IV data is combined with the previous three SK phases, the SK extracted solar neutrino flux is found to be $[2.37\pm0.015\mbox{(stat.)}\pm0.04\mbox{(syst.)}]\times10^6$/(cm$^{2}$sec). The combination of the SK recoil electron energy spectra slightly favors distortions due to a changing electron flavor content. Such distortions are predicted when assuming standard solar neutrino oscillation solutions. An extended maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the neutrino-electron elastic scattering rate results in a day-night asymmetry of $[-3.2\pm1.1$(stat.)$\pm0.5$(syst.)]$\%$. A solar neutrino global oscillation analysis including all current solar neutrino data, as well as KamLAND reactor antineutrino data, measures the solar mixing angle as $\sin^2\theta_{12}=0.305\pm0.013$, the solar neutrino mass squared splitting as $\Delta m^2_{21}=7.49^{+0.19}_{-0.17}\times10^{-5}$eV$^2$ and $\sin^2\theta_{13}=0.026^{+0.017}_{-0.012}$.