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

Abstract

The CMS and ATLAS Collaborations have reported a $t\bar{t}$ threshold enhancement possibly involving a pseudoscalar toponium quasi-bound state and an elementary pseudoscalar. Identifying the elementary pseudoscalar with the CP-odd Higgs boson, we investigate electroweak baryogenesis in the top-philic Type-III two-Higgs-doublet model (2HDM), whose scalar sector may support a strong first-order electroweak phase transition. We compare two scenarios: explicit CP violation and transitional CP violation (TCPV). In the first, a complex additional top-quark Yukawa coupling induces a spatially varying phase of the top-quark mass across the bubble wall. This generates a chiral asymmetry that electroweak sphalerons convert into baryon asymmetry. In TCPV, CP symmetry would be broken inside the bubble wall but preserved in the vacuum in the absence of explicit CP violation. A small explicit CP-violating bias would favor one of the otherwise CP-conjugate wall configurations, avoiding exact cancellation of the generated baryon asymmetry. This could allow baryogenesis without large explicit CP violation, potentially alleviating electric dipole moment constraints. Combining a one-loop finite-temperature analysis of bubble nucleation with transport calculations, we find that electroweak baryogenesis with explicit CP violation remains possible, although this conclusion depends on the thermal-resummation schemes. No viable TCPV solution is found in the parameter regions explored.