We study the electroweak phase transition in the alignment limit of the
CP-conserving two-Higgs-doublet model (2HDM) of Type I and Type II. The
effective potential is evaluated at one-loop, where the thermal potential
includes Daisy corrections and is reliably approximated by means of a sum of
Bessel functions. Both 1-stage and 2-stage electroweak phase transitions are
shown to be possible, depending on the pattern of the vacuum development as the
Universe cools down. For the 1-stage case focused on in this paper, we analyze
the properties of phase transition and discover that the field value of the
electroweak symmetry breaking vacuum at the critical temperature at which the
first order phase transition occurs is largely correlated with the vacuum depth
of the 1-loop potential at zero temperature.
We demonstrate that a strong first order electroweak phase transition
(SFOEWPT) in the 2HDM is achievable and establish benchmark scenarios leading
to different testable signatures at colliders. In addition, we verify that an
enhanced triple Higgs coupling (including loop corrections) is a typical
feature of the SFOPT driven by the additional doublet. As a result, SFOEWPT
might be able to be probed at the LHC and future lepton colliders through Higgs
pair production.Comment: 43 pages, 18 figures, minor revision and match to the published
versio
