The trap in the early Universe: impact on the interplay between gravitational waves and LHC physics in the 2HDM

TL;DR

This paper explores the thermal history of the 2HDM, focusing on phenomena like vacuum trapping and electroweak phase transitions, and discusses implications for gravitational wave detection and collider experiments.

Contribution

It reveals how vacuum trapping affects electroweak phase transition prospects and links gravitational wave signals with collider search capabilities in the 2HDM.

Findings

Vacuum trapping impedes first-order electroweak phase transition in certain 2HDM regions.

Detectable gravitational wave signals are highly constrained and correlated with collider search results.

The ILC can probe most 2HDM scenarios with a first-order phase transition via Higgs self-coupling measurements.

Abstract

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.