The trap in the early Universe: impact on the interplay between gravitational waves and LHC physics in the 2HDM
We analyze the thermal history of the 2HDM and determine the parameter regions featuring a first-order electroweak phase transition (FOEWPT) and also much less studied phenomena like high-temperature electroweak (EW) symmetry non-restoration and the possibility of vacuum trapping (i.e. the Universe remains trapped in an EW-symmetric vacuum throughout the cosmological evolution, despite at $T=0$ the EW breaking vacuum is deeper). We show that the presence of vacuum trapping impedes a first-order EW phase transition in 2HDM parameter-space regions previously considered suitable for the realization of electroweak baryogenesis. Focusing then on the regions that do feature such a first-order transition, we show that the 2HDM parameter space that would yield a stochastic gravitational wave signal potentially detectable by the future LISA observatory is very contrived, and will be well probed by direct searches of 2HDM Higgs bosons at the HL-LHC, and (possibly) also via measurements of the self-coupling of the Higgs boson at 125 GeV. This has an important impact on the interplay between LISA and the LHC regarding the exploration of first-order phase transition scenarios in the 2HDM: the absence of new physics indications at the HL-LHC would severely limit the prospects of a detection by LISA. Finally, we demonstrate that as a consequence of the predicted enhancement of the self-coupling of the Higgs boson at 125 GeV the ILC would be able to probe the majority of the 2HDM parameter space yielding a FOEWPT through measurements of the self-coupling, with a large improvement in precision with respect to the HL-LHC.