The MSSM electroweak phase transition on the lattice

We study the MSSM finite temperature electroweak phase transition with latt ice Monte Carlo simulations, for a large Higgs mass (m(H) approximate to 95 GeV) and light stop masses (m(<(tau)over bar R>) similar to 150-160 GeV). We employ a 3d effective held theory approach, where the degrees of freedom appearing in the action are the SU(2) and SU(3) gauge fields, the weakly i nteracting Higgs doublet, and the strongly interacting stop triplet. We det ermine the phase diagram, the critical temperatures, the scaler field expec tation values, the latent heat, the interface tension and the correlation l engths at the phase transition points. Extrapolating the results to the inf inite volume and continuum limits, we find that the transition is stronger than indicated by 2-loop perturbation theory, guaranteeing that the MSSM ph ase transition is strong enough for baryogenesis in this regime. We also st udy the possibility of a two-stage phase transition, in which the stop fiel d gets an expectation value in an intermediate phase, We find that a two-st age transition exists non-perturbatively, as well, but for somewhat smaller stop masses than in perturbation theory. Finally the latter stage of the t wo-stage transition is found to be extremely strong, and thus it might not be allowed in the cosmological environment. (C) 1998 Elsevier Science B.V.