WEAK INTERATIONS IN A MEDIUM & THEIR IMPLICATIONS IN ASTROPHYSICS/COSMOLOGY

Abstract

In the Standard Model of elementary particle physics neutrinos are exactly massless. However, this is an ad-hoc assumption, since the masslessness of the neutrinos is not ensured by any basic principle and observationally, one can only place limits on the neutrino mass. Non-zero neutrino masses arise in many extensions of the Standard Model. Massive neutrino and its associated properties, such as the dispersion through medium, matter induced mixing with other neutrino states and its electromagnetic interactions have important con sequences in astrophysics and cosmology. Here, we report, fo11owing three aspects of investigation of the project related with the neutrino dispersion in the medium: (i) Matter-induced spin-flavour neutrino oscillations at finite temperature and density; (ii) Neutrino oscillations and Faraday Effect; and (iii) Neutrino dispersion in a minimal SUSY model with explicitly broken R-parity at finite temperature and density. In aspect (i), we investigate the index of refraction of a weakly interacting massive Dirac neutrino and propose to study a Geld theoretic model to generalize the spin-flavour neutrino oscillations within the framework of finite temperature and density theory. We carry out the calculations in a. particular extension of the standard model by Fukugita and Yanagida. This model is designed to give maximal neutrino magnetic moment required for the above mentioned oscillations scenario. The range of the oscillations parameters for the solar neutrino problem is then predicted. Further, the applications of the result to birefringence phenomenon in the early universe are discussed. In item (ii), an investigation of an effect analogous to the Faraday Effect for a massive neutrino de Broglie wave is carried out. This study leads to a general understanding of the nature of neutrino and provides a natural and consistent choice of the oscillations parameters corresponding to different oscil1ations scenarios, like matter induced flavour oscillations (MSW effect), spin rotation induced by magnetic field ( OVV effect) and matter induced spin-flavour oscillations. A detailed numerical study for the MSW-flavour oscillation case has been reported in the framework of the Faraday Effect. Item (iii) is a supersymmetric generalization of work carried out under item (i). We extend a (vacuum) field theory supersymmetric standard model with explicitly broken R-parity to finite temperature and density, where sfermions coupled to fermions give rise to chirality conserving as well as chirality violating neutrino transitions. The refractive index of the neutrino undergoing such transitions in a heat bath of finite density is then calculated in this frame work. Some of the implications of these results to the MSW resonance and in the early universe are discussed and compared with earlier works.