Is Large Mixing Angle MSW the Solution of the Solar Neutrino Problems?

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%-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 (similar to 1%), and (5) annual variations of the signal due to the regeneration effect (similar to 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 the available SuperKamiokande data for 708 days of operation and discuss the possibilities for distinguishing experimentally between LMA and vacuum oscillations. If LMA is correct, global solutions combining data from different types of measurements made by SuperKamiokande or by different solar neutrino experiments could reveal in the next few years a many sigma indication of neutrino oscillations. [S0556-2821(99)07919-9].