Solar ν¯_e flux: revisiting bounds on neutrino magnetic moments and solar magnetic field

The interaction of neutrino transition magnetic dipole moments with magneticfields can give rise to the phenomenon of neutrino spin-flavour precession(SFP). For Majorana neutrinos, the combined action of SFP of solar neutrinosand flavour oscillations would manifest itself as a small, yet potentiallydetectable, flux of electron antineutrinos coming from the Sun. Non-observationof such a flux constrains the product of the neutrino magnetic moment $\mu$ andthe strength of the solar magnetic field $B$. We derive a simple analyticalexpression for the expected $\bar{\nu}_e$ appearance probability in thethree-flavour framework and we use it to revisit the existing experimentalbounds on $\mu B$. A full numerical calculation has also been performed tocheck the validity of the analytical result. We also present our numericalresults in energy-binned form, convenient for analyses of the data of thecurrent and future experiments searching for the solar $\bar{\nu}_e$ flux. Inaddition, we give a comprehensive compilation of other existing limits onneutrino magnetic moments and of the expressions for the probed effectivemagnetic moments in terms of the fundamental neutrino magnetic moments andleptonic mixing parameters.