Terrestrial Matter Effects from Solar Neutrino Interactions in Super-Kamiokande

Super-Kamiokande measures the rate and spectrum of recoil electrons from 8B and hep solar neutrino-electron elastic scattering as a function of the solar zenith angle. A direct search for matter effects on neutrino oscillations results in a 2.8σ indication that the electron flavor content of solar neutrino during nighttime (N), where earth matter effects are present, exceeds that of daytime (D). An unbinned maximum likelihood fit determines the amplitude of this “day/night variation” ADN=(D−N)/[½(D+N)] to be −3.3±1.0(stat)±0.5(syst)% of the average interaction rate. An oscillation analysis determines the difference in mass2 to be , the world's most precise determination of that parameter using neutrinos. Neutrinos above a few MeV are expected to undergo resonant flavor conversion in the solar core while lower energy neutrinos do not. No significant distortions of the recoil electron spectrum due to that effect were found, but the data is consistent with the prediction.