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

TL;DR

This paper evaluates the complex 2HDM as a viable framework for electroweak baryogenesis, integrating LHC Run 2 data, and explores its testability through collider experiments and gravitational wave observations, while addressing EDM constraints.

Contribution

It provides the first comprehensive analysis combining LHC constraints with baryogenesis viability in the C2HDM, including benchmark scenarios for future tests.

Findings

Identifies parameter regions compatible with electroweak baryogenesis and testable at LHC and LISA.

Highlights tension between baryogenesis requirements and electron EDM constraints.

Proposes benchmark planes to guide experimental searches for CP violation.

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.