Interpreting the 95 GeV resonance in the Two Higgs Doublet Model: Implications for the Electroweak Phase Transition

TL;DR

This study explores whether the 95 GeV resonance observed at the LHC can be explained within the Two Higgs Doublet Model and its implications for the electroweak phase transition and gravitational wave signals.

Contribution

First large-scale parameter scan of the Type I 2HDM linking the 95 GeV resonance to electroweak phase transition properties.

Findings

The 2HDM generally predicts a first-order electroweak phase transition.

Transition strength can reach up to v_c/T_C ~ 1.3, but typically less than 1 in viable models.

Gravitational wave signals are below the sensitivity of future detectors.

Abstract

We investigate if the recent mass resonance excesses seen around 95 GeV at the Large Hadron Collider (LHC) can be reconciled with a first-order electroweak phase transition. Performing the first large-scale parameter scan of the Type I Two Higgs Doublet Model (2HDM) using high-temperature dimensionally reduced effective field theory, we focus on regions of parameter space consistent with interpreting the excess as an additional pseudoscalar state. We find that, in contrast to the Standard Model, the electroweak transition pattern in the 2HDM is generically first-order, proceeding either in a single or in two steps. While transition strengths can reach up to $v_c/T_C \sim 1.3$, the viable, collider-constrained parameter space yields $v_c/T_C \lesssim 1$. Thus, the gravitational wave signals lie below the projected reach of future interferometer experiments and are likely insufficient to support successful electroweak baryogenesis.