SNO and the neutrino magnetic moment solution of the solar neutrino problem

Abstract

Assuming that the solar neutrino deficit observed in the Homestake, SAGE, Gallex, Kamiokande and Super-Kamiokande experiments is due to the interaction of the neutrino transition magnetic moment with the solar magnetic field, we calculate the expected values of a number of observables to be measured by SNO. For three model solar magnetic field profiles that produce the best fits of the previous data we calculate the charged current event rate, the (neutral current)/(charged current) event ratio and the charged current electron spectrum as well as its first and second moments. We study the dependence of the calculated observables on the choice of the magnetic field profile and on the value of the solar $hep$ neutrino flux. We also compare our results with those obtained assuming that the solar neutrino problem is due to neutrino oscillations in vacuum or in matter. We show that there is an overlap or partial overlap between our predictions and those found for each of the oscillation solutions (SMA, LMA, LOW and VO). Given the uncertainties in the calculations and the expected uncertainties in the experimental results, the unambiguous discrimination between the two types of solutions to the solar neutrino problem (neutrino oscillations and magnetic moments) on the basis of the average rates and electron spectrum distortions appears to be difficult. The possible time dependence of the charged current signal and spectrum distortion in the case of the magnetic moment solution therefore remains the best hope for such a discrimination. For a hybrid solution (neutrino magnetic moment plus flavour mixing) the smoking gun signature would be an observation of $\barν_e$'s from the sun.