Probing neutrino magnetic moment and unparticle interactions with Borexino

Abstract

We discuss the limits on the neutrino magnetic moment and hypothetical interactions with a hidden unparticle sector, coming from the first neutrino data release of the Borexino experiment. The observed spectrum in Borexino depends weakly on the solar model used in the analysis, since most of the signal comes from the monoenergetic $^{7}\mathrm{Be}$ neutrinos. This fact allows us to calibrate the $\ensuremath{\nu}\ensuremath{-}e$ scattering cross section through the spectral shape. In this way, we have derived a limit on the magnetic moment for the neutrinos coming from the Sun (in which a ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ and ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ component is present): ${\ensuremath{\mu}}_{\ensuremath{\nu}}\ensuremath{\le}8.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\ensuremath{\mu}}_{B}$ (90% CL) which is comparable with those obtained from low-energy reactor experiments. Moreover, we improve the previous upper limit on magnetic moment of the ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ by 3 orders of magnitude and the limit on the coupling constant of the neutrino with a hidden unparticle sector.