Hierarchical vs Degenerate 2HDM: The LHC Run 1 Legacy at the Onset of Run 2

TL;DR

This paper explores the implications of a non-degenerate scalar spectrum in the two-Higgs-doublet model (2HDM), highlighting its cosmological motivation and how it alters experimental search constraints compared to the degenerate case.

Contribution

It provides a detailed analysis of non-degenerate 2HDM spectra, emphasizing their cosmological significance and impact on LHC search constraints, extending beyond the commonly studied degenerate scenario.

Findings

Non-degenerate 2HDM spectra can facilitate a strongly first order electroweak phase transition.

Search constraints for new scalars are significantly different in non-degenerate spectra.

Higgs signal strength measurements remain unaffected by the spectrum degeneracy.

Abstract

Current discussions of the allowed two-Higgs-doublet model (2HDM) parameter space after LHC Run 1 and the prospects for Run 2 are commonly phrased in the context of a quasi-degenerate spectrum for the new scalars. Here we discuss the generic situation of a 2HDM with a non-degenerate spectrum for the new scalars. This is highly motivated from a cosmological perspective since it naturally leads to a strongly first order electroweak phase transition that could explain the matter-antimatter asymmetry in the Universe. While constraints from measurements of Higgs signal strengths do not change, those from searches of new scalar states get modified dramatically once a non-degenerate spectrum is considered.