Matter effects in four-neutrino mixing - art. no. 073011

The evidence in favor of neutrino oscillations implies the existence of at least three independent neutrino mass-squared differences. If the Liquid Sc intillation Neutrino Detector (LSND) results are confirmed, transitions int o active and sterile neutrinos can take place simultaneously for both solar and atmospheric neutrinos. In this paper we present a formalism for the ca lculation of the transition probabilities into active and sterile neutrinos for solar nu(e)'s and atmospheric nu(mu)'s, taking into account the matter effects in the Sun and in the Earth. We find that the solar neutrino trans ition probabilities depend on just one single additional parameter compared to that of the standard two-generation analysis, while for atmospheric neu trinos two additional mixing angles are necessary to analyze the data in ad dition to those of the usual two-generation analysis.