The CP-violating 2HDM in light of a strong first order electroweak phase transition and implications for Higgs pair production

Abstract

We investigate the strength of the electroweak phase transition (EWPT) within the CPviolating 2-Higgs-Doublet Model (C2HDM). The 2HDM is a simple and well-studied model, which can feature CP violation at tree level in its extended scalar sector. This makes it, in contrast to the Standard Model (SM), a promising candidate for explaining the baryon asymmetry of the universe through electroweak baryogenesis. We apply a renormalisation scheme which allows efficient scans of the C2HDM parameter space by using the loop-corrected masses and mixing matrix as input parameters. This procedure enables us to investigate the possibility of a strong first order EWPT required for baryogenesis and study its phenomenological implications for the LHC. Like in the CPconserving (real) 2HDM (R2HDM) we find that a strong EWPT favours mass gaps between the non-SM-like Higgs bosons. These lead to prominent final states comprised of gauge+Higgs bosons or pairs of Higgs bosons. In contrast to the R2HDM, the CP-mixing of the C2HDM also favours approximately mass degenerate spectra with dominant decays into SM particles. The requirement of a strong EWPT further allows us to distinguish the C2HDM from the R2HDM using the signal strengths of the SM-like Higgs boson. We additionally find that a strong EWPT requires an enhancement of the SM-like trilinear Higgs coupling at next-to-leading order (NLO) by up to a factor of 2.4 compared to the NLO SM coupling, establishing another link between cosmology and collider phenomenology. We provide several C2HDM benchmark scenarios compatible with a strong EWPT and all experimental and theoretical constraints. We include the dominant branching ratios of the non-SM-like Higgs bosons as well as the Higgs pair production cross section of the SM-like Higgs boson for every benchmark point. The pair production cross sections can be substantially enhanced compared to the SM and could be observable at the high-luminosity LHC, allowing access to the trilinear Higgs 1 For other recent works on the link between CP violation and electroweak baryogenesis, see [63][64][65]. 2 Recent investigations on the interplay between a strong first order phase transition and the size of the trilinear Higgs self-couplings can also be found in [66][67][68].3 Discussions on the gauge dependence of vc/Tc can be found e.g. in [70,[74][75][76].

Leading two-loop corrections to the Higgs boson self-couplings in models with extended scalar sectors

“…We should emphasise here that as these are tree-level relations, they can only be used if the masses m Φ are tree-level MS mass parameters (the relation between Lagrangian parameters and scalar masses computed at the loop level has been investigated for instance in Refs. [35,59,60,[89][90][91][92][93]).…”

High scale impact in alignment and decoupling in two-Higgs-doublet models

“…(v) It is well known that one-loop corrections to the quartic couplings can be very large in some regions of the 2HDM parameter space [97,98]. As these are then used in the matching procedure, large deviations are to be expected in the one-loop matching procedure [99,100] compared to the values obtained via treelevel matching. Therefore, substantial phenomenological differences in individual choices of parameters may be expected, but again, an overall analysis of the parameter space will yield mostly the same trends in terms of scale validity and mass bounds.…”

Exploring dynamical CP violation induced baryogenesis by gravitational waves and colliders

“…This dimension-five operator actually appears in many composite models, and this source of CP violation for BAU evolves with the scalar field S. At the very early Universe, hSi ¼ 0, 2 and then the value evolves to hSi ¼ σ through a second-order phase transition. The CP-violating top-quark Yukawa coupling is then obtained and can source the BAU in the following SFOPT [17]. After that, hSi evolves to zero again, and the CP-violating top-quark Yukawa coupling vanishes at the tree level.…”