Electroweak Phase Transition and Neutrino Masses

The presently observed cosmological baryon asymmetry has been finally determined at the time of the electroweak phase transition, when baryon and lepton number violating interactions fell out of thermal equilibrium. We discuss the thermodynamics of the phase transition based on the free energy of the SU(2) Higgs model at finite temperature, which has been studied in perturbation theory and lattice simulations. The results suggest that the baryon asymmetry has been generated by lepton number violating interactions in the symmetric phase of the standard model, i.e., at temperatures above the critical temperature of the electroweak transition. The observed value of the baryon asymmetry, $n_B/s \sim 10^{-10}$, is naturally obtained in an extension of the standard model with right-handed neutrinos where $B-L$ is broken at the unification scale \Lambda_{GUT}}\sim 10^{16} GeV. The corresponding pattern of masses and mixings of the light neutrinos $\nu_e$, $\nu_\mu$ and $\nu_\tau$ is briefly described.