Is a large mixing angle MSW effect the solution of the solar neutrino problems?

Abstract

Recent results on solar neutrinos provide hints that the LMA MSW solution could be correct. We perform accurate calculations for potential `smoking-gun' effects of the LMA solution in the SuperKamiokande solar neutrino experiment, including: (1) an almost constant reduction of the standard recoil electron energy spectrum (with a weak, < 10%, relative increase below 6.5 MeV); (2) an integrated difference in day-night rates (2% to 14%); (3) an approximately constant zenith-angle dependence of the nighttime event rate; (4) a new test for the difference in the shape of the equally-normalized day-night energy spectra (~ 1%); and (5) annual variations of the signal due to the regeneration effect (~ 6 times smaller than the integrated day-night effect). We also establish a relation between the integrated day-night asymmetry and the seasonal asymmetry due to LMA regeneration. As a cautionary example, we simulate the effect of an absolute energy calibration error on the shape (distortion) of the recoil energy spectrum. We compare LMA predictions with SuperKamiokande data and discuss the possibilities for distinguishing experimentally between LMA and vacuum oscillations.