Electroweak phase transition in singlet extensions of the standard model with dimension-six operators

Abstract

The significance of the electroweak phase transition is undeniable, and although initially it was believed that it was second order, it is now believed that it is a first-order transition. However, it is not a strong first-order phase transition in the context of the standard model, and the remedy to this issue is to use the Higgs portal and directly couple the Higgs to a hidden scalar sector. This can result in a strong electroweak phase transition, while the couplings to a hidden scalar are constrained by several phenomenological constraints, such as the sphaleron rate criterion and the branching ratio of the Higgs to invisible channels. In this work, we consider the standard singlet extensions of the standard model, including dimension-six non-renormalizable operators that couple a real singlet scalar field with the Higgs doublet. As a result, we examine the effects of those Higgs-singlet couplings on the electroweak phase transition. The effective theory, where the nonrenormalizable couplings originate from, is considered to be active beyond 15 TeV. As we show, the Universe experiences a two-step electroweak phase transition, a primary phase transition in the singlet sector at a high temperature, and then a subsequent first-order phase transition from the singlet vacuum to the electroweak vacuum. The singlet’s phase transition can either be second order or first order, depending on the singlet mass and its couplings to the Higgs. In particular, we show that the dimension-six operator assists in generating a strong electroweak phase transition in regions of the parameter space that were excluded in the previous singlet extensions of the standard model. This is further apparent for low singlet masses mS<mH/2, which are rarely taken into account in the literature due to the invisible branching ratio of the Higgs boson. In some limited cases in the parameter space, the electroweak phase transition is weakened by the presence of the higher order operator. Published by the American Physical Society 2024