Singlet-Catalyzed Electroweak Phase Transitions and Precision Higgs Studies

TL;DR

This work examines a minimal extension of the SM with a real scalar singlet (xSM) to realize a strong first-order electroweak phase transition while focusing on a regime with no new on-shell decays. It analyzes how Higgs mixing, Higgs self-coupling deviations, and precision electroweak observables constrain the parameter space, and it maps how future colliders could probe regions that yield a strong EWPT. By combining analytic considerations with CosmoTransitions-based finite-temperature calculations, it identifies correlations among portal couplings, mixing, and scalar masses that favor a strong transition and outlines observable signatures in Higgs properties and direct h2 searches. The results highlight a promising interplay between collider phenomenology and early-universe phase transition dynamics, offering a path to test EWBG-related scenarios with upcoming experiments.

Abstract

We update the phenomenology of gauge singlet extensions of the Standard Model scalar sector and their implications for the electroweak phase transition. Considering the introduction of one real scalar singlet to the scalar potential, we analyze present constraints on the potential parameters from Higgs coupling measurements at the Large Hadron Collider (LHC) and electroweak precision observables for the kinematic regime in which no new scalar decay modes arise. We then show how future precision measurements of Higgs boson signal strengths and Higgs self-coupling could probe the scalar potential parameter space associated with a strong first-order electroweak phase transition. We illustrate using benchmark precision for several future collider options, including the High Luminosity LHC (HL-LHC), the International Linear Collider (ILC), TLEP, China Electron Positron Collider (CEPC), and a 100 TeV proton-proton collider, such as the Very High Energy LHC (VHE-LHC) or the Super proton-proton Collider (SPPC). For the regions of parameter space leading to a strong first order electroweak phase transition, we find that there exists considerable potential for observable deviations from purely Standard Model Higgs properties at these prospective future colliders.

Figures (4)

• Figure 1: Left panel: $\chi^2$ fit to mixing angle from current Higgs measurements at the LHC. The current 95% C.L. limit is $|\cos \theta| \geq 0.84$. Right panel: The combined current and projected constraints on the mixing angle and singlet-like eigenstate mass $m_2$. The current limits are from a combined fit to current ATLAS and CMS Higgs measurements (black solid line), CMS heavy SM-like Higgs searches (blue region), null results from LHC SM Higgs searches (red region), null results from LEP SM Higgs searches (green region), and electroweak precision observables (beige region). The projected constraints on the mixing angle are from the $\sqrt{s}=14 \,{\rm TeV}$, $3 \; ab^{-1}$ HL-LHC run (black dashed line), $\sqrt{s}=250 \,{\rm GeV}$, $250 \; fb^{-1}$ (ILC-1) ILC run (blue solid line), $\sqrt{s}=1 \,{\rm TeV}$, $1\; ab^{-1}$ (ILC-3) ILC run (blue dashed line), and $\sqrt{s}=240 \,{\rm GeV}$, $1\; ab^{-1}$ TLEP run (red solid line).
• Figure 2: Scatter plots of the parameter space. Orange (light) points satisfy limits from current LHC measurements of Higgs properties, heavy/light SM-like Higgs searches, and electroweak precision bounds. Black points further satisfy the requirement of a SFOEWPT and exhibit a sufficiently fast thermal tunnelling rate to support bubble nucleation.
• Figure 3: Top Row: Scatter plots showing the effect of the more stringent bounds on the mixing angle from the HL-LHC and ILC-3 accelerator programs. Bottom Row: Distributions of EWPT-preferred points overlayed with collider and electroweak precision bounds for the current LHC (left), HL-LHC (middle), and ILC-3 (right).
• Figure 4: Correlation between the SM-like scalar $(h_1)$ self-coupling $g_{111}$ and the critical temperature for SFOEWPT-viable parameter space points. Blue, red, green, and yellow bands represent, respectively, a $\pm$50%, $\pm$30%, $\pm$13%, and $\pm$5% variation in $g_{111}$ about its SM value.