Electroweak Phase Transition in Two Higgs Doublet Models

We reexamine the strength of the first order phase transition in the electroweak theory supplemented by an extra Higgs doublet. The finite-temperature effective potential, $V_{eff}$, is computed to one-loop order, including the summation of ring diagrams, to study the ratio $\phi_c/T_c$ of the Higgs field VEV to the critical temperature. We make a number of improvements over previous treatments, including a consistent treatment of Goldstone bosons in $V_{eff}$, an accurate analytic approximation to $V_{eff}$ valid for any mass-to-temperature ratios, and use of the experimentally measured top quark mass. For two-Higgs doublet models, we identify a significant region of parameter space where $\phi_c/T_c$ is large enough for electroweak baryogenesis, and we argue that this identification should persist even at higher orders in perturbation theory. In the case of the minimal supersymmetric standard model, our results indicate that the extra Higgs bosons have little effect on the strength of the phase transition.