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

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.

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