The Solar Neutrino Problem after Three Hundred Days of Data at Superkamiokande Typeset Using Revt E X 1

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 Su-perKamiokande experiment during 306.3 days of operation. In particular, the observed energy spectrum of the recoil electrons from 8 B 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 semi-annual modulation of the 8 B ux due to vacuum oscillations should require at least one more year of operation of SuperKamiokande.