Four-neutrino mixing and Big-Bang Nucleosynthesis

We investigate the Big-Bang Nucleosynthesis constraints on the elements of the neutrino mixing matrix which connect sterile with massive neutrino fiel ds, in the framework of the two four-neutrino schemes that are favored by t he results of neutrino oscillation experiments, We discuss the implications of these constraints for terrestrial short and long-baseline neutrino osci llation experiments and we present several possibilities of testing them in these experiments. In particular, we show that from the Big-Bang Nucleosyn thesis constraints it follows that the ((-))nu(mu) --> ((-))nu(tau) transit ion is severely suppressed in short-baseline experiments, whereas its oscil lation amplitude in long-baseline experiments is of order i. We also propos e a new parameterization of the four-neutrino mixing matrix U which is appr opriate for the schemes under consideration. (C) 1999 Elsevier Science B.V. All rights reserved.