General properties of neutrino spin precession in magnetic field and their possible implication for solar neutrinos.

The neutrino decay solution of the solar neutrino problem is revisited in the context of majoron models. It is shown that for a definite range of parameters this scenario reconciles both the Homestake and the Kamiokande data. The prediction for Gallium detectors is also given. Attention is devoted to the analysis of the $$\bar v_e $$ signal, which is the crucial prediction of this scenario. It is shown that the sensitivity of Borexino is sufficient to observe this signal and to distinguish it from the alternative $$\bar v_e $$ signal provided by hybrid models of neutrino oscillation and magnetic moment transitions. Other sources of solar $$\bar v_e $$ production are also reviewed (matter induced decay, MSW catalized decay or the decay of solar 17 keV neutrinos)