Electroweak Baryogenesis in Top-Philic Type-III Two-Higgs-Doublet Model motivated by the $t\bar{t}$ Excess at the LHC

Abstract

Recently, the CMS and ATLAS Collaborations reported an enhancement near the production threshold in the invariant-mass distribution of top-antitop pairs. Possible interpretations include a pseudoscalar toponium quasi-bound state and an additional elementary pseudoscalar that coexists or mixes with toponium. Motivated by the latter interpretation, we identify the additional pseudoscalar with the CP-odd Higgs boson of a top-philic Type-III two-Higgs-doublet model (2HDM). This possibility is also interesting from the viewpoint of electroweak baryogenesis: the extended scalar sector can support a strong first-order electroweak phase transition, while the additional top-quark Yukawa interaction can provide a CP-violating source through the bubble-wall background. We therefore investigate whether the same parameter region motivated by the $t\bar{t}$ threshold enhancement can also account for the observed baryon asymmetry of the Universe (BAU). We compare two scenarios with different origins of CP violation. In the explicit CP violation scenario, a complex phase is introduced into the additional top-quark Yukawa coupling $\rho_{tt}$. In the transitional CP violation (TCPV) scenario, a CP-odd field configuration is generated dynamically inside the bubble wall through finite-temperature effects. Within the present one-loop treatment, explicit CP violation remains a scheme-dependent possibility, whereas no viable transitional CP violation branch is identified. In addition, we discuss the physical differences and viability of these two scenarios.