Solar neutrinos in Super-Kamiokande

AbstractRecently the concern with the effect of matter on the neutrino oscillation has been growing, because the possibility of mass hierarchy determination by next generation experiments through the matter effect has been recognized. We report an indication that the elastic scattering rate of solar 8B neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through Earth during nighttime. This is the first direct indication that neutrino oscillation probabilities are modified by the presence of matter. The observed the day-night asymmetry is −3.3±1.0(stat.)±0.5(syst.) for Δm212=4.84×105eV2, sin2⁡θ12=0.311 and sin2⁡θ13=0.025, which deviates from zero by 3σ. Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a nonzero day-night asymmetry implies that the flavor oscillations of solar neutrinos are affected by the presence of matter within the neutrinos' flight path. Super-Kamiokande's day-night asymmetry is consistent with neutrino oscillations for 4×10−5eV2≤Δm212≤7×10−5eV2 and large mixing values of θ12 at the 68% C.L. We also report the measured recoil electron spectrum whose shape should reflect the transition between vacuum dominated oscillations (lower energy solar neutrinos) and matter dominated oscillations (higher energy solar neutrinos)