Four-Neutrino Mixing, Oscillations and Big-Bang Nucleosynthesis

S.M. BilenkyJoint Institute for Nuclear Research, Dubna, Russia, and Insti-tut fu¨r Theoretische Physik, Technische Universitat Munchen, D–85748 Garching, GermanyC. GiuntiINFN, Sezione di Torino, and Dipartimento di Fisica Teorica, Uni-versita di Torino, Via P. Giuria 1, I–10125 Torino, ItalyW. Grimus and T. SchwetzInstitute for Theoretical Physics, University of Vienna, Boltzman-ngasse 5, A–1090 Vienna, AustriaAbstractWe investigate the implications of the standard Big-Bang Nucleosynthesisconstraint on the number of light neutrinos in the framework of the two four-neutrino schemes that are favored by the results of neutrino oscillation experi-ments.Talk presented by C. Giunti at the Symposium ”New Era in Neutrino Physics”,Tokyo Metropolitan University, Japan, 11-12 June, 1998.DFTT 51/98, TUM-HEP-328/98, SFB 375-308.1. IntroductionForty years after Pontecorvo’s discovery of the theory of neutrino oscilla-tions [1], there are today three experimental indications in favor of the existenceof this phenomenon. The strongest evidence is believed to come from the at-mospheric neutrino anomaly observed by the Super-Kamiokande experiment [2],following the previous measurements of the Kamiokande, IMB and Soudan at-mospheric neutrino experiments [3]. The atmospheric neutrino anomaly consistsin a suppression of the ratio of measured atmospheric ν