Medium-induced neutrino decays: Physical and astrophysical effects

It is well known that the medium effects can substantially modify the picture of neutrino oscillations. We demonstrated that the coherent interaction of neutrinos with matter could be important for their decays also: new transitions become possible which are principally forbidden in vacuum. In particular, neutrino (even massless) can decay into antineutrino with emission of light scalar particle of the majoron type, or vice versa depending on neutrino type and chemical composition of the medium. The diagonal as well as non-diagonal transitions on neutrino types are possible. Properties of these decays are analysed in some details. The decay probabilities depend on neutrino energies in an anusual way. Applications for (1) accelerator neutrinos passing through the Earth (2) Solar neutrinos (3) gravitational collapse of the stellar core and the accompanied neutrino burst are discussed. Strong constraints on the neutrino-majoron coupling constants are obtained from the SN 1987A data