Limits on the Neutrino Magnetic Moment using 1496 Days of Super-Kamiokande-I Solar Neutrino Data

A search for a nonzero neutrino magnetic moment has been conducted using 1496 live days of solar neutrino data from Super-Kamiokande-I. Specifically, we searched for distortions to the energy spectrum of recoil electrons arising from magnetic scattering due to a nonzero neutrino magnetic moment. In the absence of a clear signal, we found ${\mu }_{\nu }\le (3.6\times {10}^{-10}){\mu }_B$ at 90% C.L. by fitting to the Super-Kamiokande day-night spectra. The fitting took into account the effect of neutrino oscillation on the shapes of energy spectra. With additional information from other solar neutrino and KamLAND experiments constraining the oscillation region, a limit of ${\mu }_{\nu }\le (1.1\times {10}^{-10}){\mu }_B$ at 90% C.L. was obtained.

Figure 1

Ratio of SK-I observed recoil electron energy spectrum and the expected nonoscillated weak scattering spectrum. The error bars are the results of the statistical and energy noncorrelated systematic errors being added in quadrature. The dotted lines are the correlated systematic errors. The dash-dotted line is the expected oscillated weak scattering spectrum for $\Delta m^2=6.6\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta =0.48$. The dashed line shows the addition of magnetic scattering with ${\mu }_{\nu }\le (1.1\times {10}^{-10}){\mu }_B$ on top of the oscillated weak spectrum. (The zero has been suppressed).

Figure 2

The $95\%$ C.L. exclusion regions using the SK day-night spectra shape. The shaded area assumes only weak scattering. The hatched region takes into account the contribution from magnetic scattering.

Figure 3

The probability distribution of $\Delta {\chi }^2$ as a function of ${\mu }_{\nu }^2$. The solid line is for the case of no oscillation. The dashed line is for $\Delta m^2=2.8\times {10}^{-5}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta =0.42$ (LMA). The dash-dotted line is for $\Delta m^2=3.13\times {10}^{-11}{\mathrm{e}\mathrm{V}}^2$ and ${tan}^2\theta =0.91$ (VAC). The arrows point to the place where the 90% C.L. limits are. The hatched areas to the right of the arrows are 10% of the total areas under the curves.

Figure 4

The 90% C.L. ${\mu }_{\nu }$ limit contours (in units of ${10}^{-10}{\mu }_B$) and neutrino oscillation allowed regions. The area within the dashed lines is the solar neutrino experiments' allowed region considering both weak and magnetic scattering. The shaded area shows the allowed region for solar experiments plus KamLAND.