Measurement of the solar neutrino interaction rate below 3.49 MeV in Super-Kamiokande-IV

Abstract

Super-Kamiokande (SK) has observed $^{8}\text{B}$ solar neutrino elastic scattering at recoil electron kinetic energies ($E_{\text{kin}}$) as low as 3.49 MeV to study neutrino flavor conversion within the Sun. At SK-observable energies, these conversions are dominated by the Mikheyev-Smirnov-Wolfenstein (MSW) effect. An upturn in the electron neutrino survival probability in which vacuum neutrino oscillations become dominant is predicted to occur at lower energies, but radioactive background increases exponentially with decreasing energy. New machine learning approaches provide substantial background reduction below 3.49 MeV such that statistical extraction of solar neutrino interactions becomes feasible. This article presents an analysis of the solar neutrino interaction rate at $E_{\text{kin}}$ < 3.49 MeV with the full SK-IV period, using data from a wideband intelligent trigger when available and with a boosted decision tree for event selection. A solar neutrino signal is observed between 2.99 MeV < $E_{\text{kin}}$ < 3.49 MeV with $2.76σ$ significance and a data to unoscillated Monte Carlo ratio of $0.307^{+0.112}_{-0.111}$. These additional low-energy data have a negligible effect on the $1σ$ intervals of the fits to the solar neutrino energy spectrum but have a noticeable effect on the best fit when using the exponential parametrization.