Four-neutrino oscillation solutions of the solar neutrino problem -: art. no. 013005

We present an analysis of the neutrino oscillation solutions of the solar neutrino problem in the framework of four-neutrino mixing where a sterile neutrino is added to the three standard ones. We perform a nt to the full data set corresponding to the 825-day Super-Kamiokande data sample as well as to chlorine, GALLEX, and SAGE and Kamiokande experiments. In our analysis we use all measured total event rates as well as all Super-Kamiokande data on the zenith angle dependence and the recoil electron energy spectrum. We consider both transitions via the Mikheyev-Smirnov-Wolfenstein (MSW) mechanism as well as oscillations in vacuum (just-so) and find the allowed solutions for different values of the additional mixing angles. This framework permits transitions into active or sterile neutrinos controlled by the additional parameter cos(2)(theta(23))cos(2)(theta(24)) and contains as limiting cases the pure nu(e)-active and nu(e)-sterile neutrino oscillations. We discuss the maximum allowed values of this additional mixing parameter for the different solutions. As a particularity, we also show that for MSW transitions there are solutions at 99% C.L. at theta(12) mixing angles greater than pi/4 and that the best-fit point for the zenith angle distribution is in the second octant.