First Indication of Terrestrial Matter Effects on Solar Neutrino Oscillation

We report an indication that the elastic scattering rate of solar ${}^8\mathrm{B}$ neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through 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 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 $4\times 10^{-5}{\mathrm{eV}}^2\le \mathrm{\Delta }{m}_{21}^2\le 7\times 10^{-5}{\mathrm{eV}}^2$ and large mixing values of ${\theta }_{12}$, at the 68% C.L.

Figure 1

SK combined solar zenith angle dependence of the ${}^8\mathrm{B}$ solar neutrino flux. Solid red (dashed blue) gives the prediction based on oscillation parameters from a fit to SK data while constraining the flux ($\text{solar}+\text{KamLAND}$ fit) and the dashed-dotted line gives the total average flux.

Figure 2

SK day-night amplitude fit as a function of recoil electron kinetic energy (unlike [16] which uses total energy), shown as the measured amplitude times the expected day-night asymmetry, for oscillation parameters chosen by the SK best fit. The error bars shown are statistical uncertainties only, and the expected dependence is shown in red.

Figure 3

Dependence of the measured day-night asymmetry [fitted day-night amplitude times the expected day-night asymmetry (red)] on $\mathrm{\Delta }{m}_{21}^2$, for ${\sin }^2{\theta }_{12}=0.314$, and ${\sin }^2{\theta }_{13}=0.025$. The $1\sigma$ statistical uncertainties are given by the light gray band. The additional dark gray width to the band shows the inclusion of the systematic uncertainties. Overlaid are the $1\sigma$ allowed ranges from the solar global fit (solid green) and the KamLAND experiment (dashed blue).

Figure 4

Contours at 68% (solid thin line), 95% (solid thick line), three sigma (dashed-dotted line), and five sigma (dashed gray line) of the flux-independent SK solar neutrino oscillation analysis. The shaded gray area indicates regions preferred by the day-night variation data for the 95% case. The best-fit parameters resulting from a fit to all solar neutrino [2, 8, 10, 11] and KamLAND [21] data is shown by the black asterisk.