Possibility of multi-step electroweak phase transition in the two Higgs doublet models

TL;DR

This paper investigates the conditions under which multi-step electroweak phase transitions occur in two Higgs doublet models, exploring their implications for baryogenesis, gravitational waves, and collider observables.

Contribution

It identifies parameter regions favoring multi-step EWPTs in 2HDMs, including specific mass hierarchies and symmetries, and computes related Higgs couplings and gravitational wave signals.

Findings

Multi-step EWPTs occur in specific parameter regions of 2HDMs.

Mass hierarchy between scalar bosons influences the order of phase transitions.

Predicted gravitational wave signals could be observed in future experiments.

Abstract

We discuss whether a multi-step electroweak phase transition (EWPT) occurs in two Higgs doublet models (2HDMs). The EWPT is related to interesting phenomena such as baryogenesis and a gravitational wave from it. We examine parameter regions in CP-conserving 2HDMs and find certain areas where the multi-step EWPTs occur. The parameter search shows the multi-step EWPT prefers the scalar potential with the approximate $Z_2$ symmetry and a mass hierarchy between the neutral CP-odd and CP-even extra scalar bosons $m_A<m_H$. By contrast, the multi-step EWPT whose first step is strongly first order favors a mass hierarchy $m_A>m_H$. In addition, we compute the Higgs trilinear coupling in the parameter region where the multi-step EWPTs occur, which can be observed at future colliders. We also discuss a multi-peaked gravitational wave from a multi-step EWPT.