Neutrinos in particle physics and cosmology

One of the outstanding problems in particle physics is the understanding of the origin of the fermion masses and mixings. In particular the explanation of the smallness of neutrino masses and the largeness of the mixing angles is of great importance. We propose a model for generating small Majorana neutrino masses at the TeV scale. The smallness of the neutrino masses in this model is not due to the largeness of the right handed neutrino mass, but to the smallness of the Dirac masses which are induced at the one loop level. The generation of the baryon asymmetry of the universe through the lepton asymmetry is also discussed within this model. The mixing angle in the leptonic sector is studied using triangular mass matrices for the charged leptons and the neutrinos. We found that the large mixing angles in both the atmospheric and solar neutrino anomalies can be obtained within these textures. Motivated by the unification of the b quark and the τ lepton in SO(10) we investigated the ansatz TrM_ν = 0. From the information of two neutrino squared mass differences and the leptonic mixing matrix one can reconstruct the neutrino mass matrix. Using the accurately known value of the Z width we investigated in detail the coupling of two neutrinos to the Majoron. Bounds on the coupling constants are obtained in both the “3 + 1” and “(3 + 1 + N)” dimensional case, with the Majoron free to travel in the N compact extra dimensions.