First Indication of Terrestrial Matter Effects on Solar Neutrino Oscillation

Abstract

We report an indication that the elastic scattering rate of solar ${}^8$B neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through the Earth during nighttime. We determine the day/night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be $(-3.2\pm 1.1(\text{stat})\pm 0.5(\text{syst}))\%$, which deviates from zero by 2.7 $\sigma$. Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a non-zero 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 $4\times 10^{-5}$eV${}^2\le \Delta m_{21}^2\le 7\times 10^{-5}$eV${}^2$ and large mixing values of ${\theta }_{12}$, at the $68\%$ C.L.

6 pages, 4 figures

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