The solar neutrino problem after three hundred days of data at SuperKamiokande

We present an updated analysis of the solar neutrino problem in terms of both Mikheyev-Smirnov-Wolfenstein (MSW) and vacuum neutrino oscillations, with the inclusion of the preliminary data collected by the SuperKamiokande experiment during 306.3 days of operation. In particular, the observed energy spectrum of the recoil electrons from B-8 neutrino scattering is discussed in detail and is used to constrain the mass-mixing parameter space. It is shown that (1) the small mixing MSW solution is preferred over the large mixing one; (2) the vacuum oscillation solutions are strongly constrained by the energy spectrum measurement; and (3) the detection of a possible semiannual modulation of the B-8 v flux due to vacuum oscillations should require at least one more year of operation of SuperKamiokande. (C) 1998 Elsevier Science B.V.