Benchmarking a fading window: electroweak baryogenesis in the C2HDM, LHC constraints after Run 2 and prospects for LISA

Abstract

The origin of the baryon asymmetry of the universe remains one of the most pressing open questions in particle physics and cosmology. Electroweak baryogenesis offers an experimentally testable explanation, requiring new sources of CP violation and a strong first-order electroweak phase transition. The Two Higgs doublet model (2HDM) is the simplest scalar extension of the Standard Model that can accommodate both ingredients. We critically assess the viability of the complex 2HDM (C2HDM) (a 2HDM with a softly broken $\mathbb{Z}_2$ symmetry and a single source of explicit CP violation in the Higgs sector) as a framework for electroweak baryogenesis, incorporating for the first time a comprehensive set of LHC Run 2 results at 13 TeV. By defining CP-violating benchmark planes tailored for a strong first-order electroweak phase transition, we identify regions of parameter space motivated by electroweak baryogenesis that will be testable at the LHC, and at future space-based gravitational wave experiments. The benchmark planes are intended to guide ongoing efforts in defining representative scenarios for the exploration of CP-violation in extended scalar sectors at the LHC Run 3 and beyond, while also assessing the emerging synergy between the LHC and future gravitational wave observatories such as LISA. We also quantify the current tension between the realisation of electroweak baryogenesis and the non-observation of the electron electric dipole moment (EDM), finding that the predicted electron EDMs typically exceed the experimental limits by at least an order of magnitude.