Homestake result, sterile neutrinos and low energy solar neutrino experiments

Abstract

The Homestake result is about ~ 2 σlower than the Ar-production rate, Q_{Ar}, predicted by the LMA MSW solution of the solar neutrino problem. Also there is no apparent upturn of the energy spectrum (R \equiv N_{obs}/N_{SSM}) at low energies in SNO and Super-Kamiokande. Both these facts can be explained if a light, Δm^2_{01} ~ (0.2 - 2) \cdot 10^{-5} eV^2, sterile neutrino exists which mixes very weakly with active neutrinos: \sin^2 2α~ (10^{-5} - 10^{-3}). We perform both the analytical and numerical study of the conversion effects in the system of two active neutrinos with the LMA parameters and one weakly mixed sterile neutrino. The presence of sterile neutrino leads to a dip in the survival probability in the intermediate energy range E = (0.5 - 5) MeV thus suppressing the Be, or/and pep, CNO as well as B electron neutrino fluxes. Apart from diminishing Q_{Ar} it leads to decrease of the Ge-production rate and may lead to decrease of the BOREXINO signal and CC/NC ratio at SNO. Future studies of the solar neutrinos by SNO, SK, BOREXINO and KamLAND as well as by the new low energy experiments will allow us to check this possibility. We present a general analysis of modifications of the LMA energy profile due to mixing with new neutrino states.