New constraints on neutrino oscillations in vacuum as a possible solution of the solar neutrino problem.

Two-neutrino oscillations in vacuum are studied as a possible solution of the solar neutrino problem. New constraints on the parameter sin 2θ, characterizing the mixing of the electron neutrino with another active or sterile neutrino, as well as on the mass–squared difference, ∆m2, of their massive neutrino components, are derived using the latest results from the four solar neutrino experiments. Oscillations into a sterile neutrino are ruled out at 99% C.L. by the observed mean event rates even if one includes the uncertainties of the standard solar model predictions in the analysis. Permanent address: Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, BG–1784 Sofia, Bulgaria.