The 125 GeV Higgs and Electroweak Phase Transition Model Classes

Recently, the ATLAS and CMS detectors have discovered a bosonic particle which, to a reasonable degree of statistical uncertainty, fits the profile of the Standard Model Higgs. One obvious implication is that models which predict a significant departure from Standard Model phenomenology, such as large exotic (e.g., invisible) Higgs decay or mixing with a hidden sector scalar, are already ruled out. This observation threatens the viability of electroweak baryogenesis, which favors, for example, a lighter Higgs and a Higgs coupled to or mixed with light scalars. To assess the broad impact of these constraints, we propose a scheme for classifying models of the electroweak phase transition and impose constraints on a class-by-class basis. We find that models, such as the MSSM, which rely on thermal loop effects are severely constrained by the measurement of a 125 GeV Higgs. Models which rely on tree-level effects from a light singlet are also restricted by invisible decay and mixing constraints. Moreover, we find that the parametric region favored by electroweak baryogenesis often coincides with an enhanced symmetry point with a distinctive phenomenological character. In particular, enhancements arising through an approximate continuous symmetry are phenomenologically disfavored, in contrast with enhancements from discrete symmetries. We also comment on the excess of diphoton events observed by ATLAS and CMS. We note that although Higgs portal models can accommodate both enhanced diphoton decay and a strongly first order electroweak phase transition, the former favors a negative Higgs portal coupling whereas the latter favors a positive one, and therefore these two constraints are at tension with one another.