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

Ansh Bhatnagar, Djuna Croon, Philipp Schicho

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.

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

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 , the viable, collider-constrained parameter space yields . Thus, the gravitational wave signals lie below the projected reach of future interferometer experiments and are likely insufficient to support successful electroweak baryogenesis.

Paper Structure

This paper contains 18 sections, 43 equations, 3 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Two Higgs Doublet Model
  3. The Type I 2HDM thermal potential
  4. The dimensionally reduced effective potential
  5. Higher-orders in the effective potential
  6. Bubble Nucleation and Gravitational Waves
  7. Bubble nucleation
  8. Gravitational Wave spectrum
  9. From dimensionally reduced potential to gravitational waves
  10. Parameter scans
  11. Scanning the parameter space for first-order phase transitions
  12. Uncertainties from higher-order corrections in the effective potential
  13. Combining collider constraints with first-order phase transition scans
  14. Conclusions
  15. Renormalisation of the Type I 2HDM
  16. ...and 3 more sections

Figures (3)

  • Figure 1: Illustrative phase diagrams for classes 1S and 2S from bands in fig. \ref{['fig:multiplot']} and benchmark points in table \ref{['tab:2loop-benchmarks']}. Their corresponding regions are shown in fig. \ref{['fig:multiplot']} all at [3D@LO]. Class (2S) exhibits a two-step transition. Class (1S) at 1-loop features a direct transition $2'$ to the zero-temperature minimum, as well a weaker transition $1’$ at a lower $T_p$. Class (1S) at 2-loop features a direct transition $1’$, followed by a crossover to the zero-temperature minimum.
  • Figure 2: Top left: First-order phase transitions in the $m_{\hbox{\tiny\rm{$H$}}}$--$\cos(\beta-\alpha)$ plane. Color indicates the largest $v_{\rm c}/T_{\rm c}$ at each point. Two-step (2S), one-step (1S), and crossover regions are labeled according to fig. \ref{['fig:phase-diagram']}. Top right: Peak amplitude $\Omega_\text{gw}h^2(f_\text{gw})$. Bottom left: Peak frequency $f_\text{gw}$. Bottom right: Phase transitions plotted against $\bar{\alpha}$ and $\bar{\beta}/H_*$ (randomly sampled subset, 1 in 4, shown for clarity). Color indicates $c_{\beta-\alpha}$; circle size indicates $m_{\hbox{\tiny\rm{$H$}}}$. LISA SNR contours assume $T_\text{p} = 160$ GeV and $v_{w} = 0.63$. The gray region indicates weak 2S transitions.
  • Figure 3: First-order phase transitions in the $m_{\hbox{\tiny\rm{$H$}}}$--$\cos(\beta-\alpha)$ plane, showing transition strength $v_{\rm c}/T_{\rm c}$ (top left), peak amplitude $\Omega_\text{gw}h^2(f_\text{gw})$ (top right), peak frequency $f_\text{gw}$ (bottom left), and phase transitions plotted against $\bar{\alpha}$ and $\bar{\beta}/H_*$ (bottom right). As in fig. \ref{['fig:multiplot']}, but with the scan following table \ref{['tab:scan2']} and applying constraints from ScannerSMuhlleitner:2016mztMuhlleitner:2020wwk.