Improving LMA predictions with non-standard interactions: neutrino decay in solar matter?

It has been known for some time that the well established LMA solution to the observed solar neutrino deficit fails to predict a flat energy spectrum for SuperKamiokande as opposed to what the data indicates. It also leads to a Chlorine rate which appears to be too high as compared to the data. We investigate the possible solution to these inconsistencies with non standard neutrino interactions, assuming that they come as extra contributions to the $\nu_{\alpha}\nu_{\beta}$ and $\nu_{\alpha}e$ vertices that affect both the propagation of neutrinos in the sun and their detection. We find that, among the many possibilities for non standard couplings, only the diagonal imaginary ones lead to a solution to the tension between the LMA predictions and the data, implying neutrino instability in the solar matter. Unitarity requirements further restrict the solution and a neutrino decay into an antineutrino and a majoron within the sun is the one favoured. Its rate lies within the Borexino upper bound for antineutrinos.