The 125 GeV Higgs and Electroweak Phase Transition Model Classes

TL;DR

The paper classifies electroweak phase transition (EWPT) models by the mechanism generating the barrier in the finite-temperature potential $V_{ m eff}(h,T)$ and analyzes how a 125 GeV Higgs constrains each class. It shows that thermally driven (BEC) and continuous-symmetry enhanced (EcSP) scenarios are strongly disfavored by collider data, while discrete-symmetry enhanced (EdSP) regions can remain viable with careful parameter choices. It also highlights tension between achieving a strongly first-order EWPT and explaining potential diphoton excesses through Higgs portal interactions, suggesting that minimal single-operator portals are unlikely to satisfy both requirements. Overall, the study provides a structured framework to evaluate EWBG viability under current Higgs measurements and LHC constraints, indicating that successful models may require additional beyond-the-portal dynamics.

Abstract

Recently, the ATLAS and CMS detectors have discovered a bosonic particle which, to a reasonable degree of statistical uncertainty, fits the profile of the Standard Model Higgs. One obvious implication is that models which predict a significant departure from Standard Model phenomenology, such as large exotic (e.g., invisible) Higgs decay or mixing with a hidden sector scalar, are already ruled out. This observation threatens the viability of electroweak baryogenesis, which favors, for example, a lighter Higgs and a Higgs coupled to or mixed with light scalars. To assess the broad impact of these constraints, we propose a scheme for classifying models of the electroweak phase transition and impose constraints on a class-by-class basis. We find that models, such as the MSSM, which rely on thermal loop effects are severely constrained by the measurement of a 125 GeV Higgs. Models which rely on tree-level effects from a light singlet are also restricted by invisible decay and mixing constraints. Moreover, we find that the parametric region favored by electroweak baryogenesis often coincides with an enhanced symmetry point with a distinctive phenomenological character. In particular, enhancements arising through an approximate continuous symmetry are phenomenologically disfavored, in contrast with enhancements from discrete symmetries. We also comment on the excess of diphoton events observed by ATLAS and CMS. We note that although Higgs portal models can accommodate both enhanced diphoton decay and a strongly first order electroweak phase transition, the former favors a negative Higgs portal coupling whereas the latter favors a positive one, and therefore these two constraints are at tension with one another.

Figures (7)

• Figure 1: The four methods of obtaining a strongly first order phase transition by inducing a barrier in the thermal effective potential, which are discussed in this paper. The framed expressions indicate which term is responsible for the rise or fall of $V_{\rm eff}$.
• Figure 2: A plot of the EW order parameter $v(T_c)/T_c$ calculated analytically (black, dashed) and numerically (black, solid), as discussed in the text, as well as modifications to Higgs production by gluon fusion (red) and Higgs decay to two photons (purple). The numeric calculation of $v(T_c) / T_c$ falls short of the analytic estimate due to the Boltzmann suppression effect discussed in the text. The analytic expression suggests that SFOPT are obtained for $Q \gtrsim 1.2$, but numerical calculation reveals that SFOPT are only found for $Q \gtrsim 1.7$. Hence, there is a narrow window $1.7 \lesssim Q \lesssim 2.0$ where the perturbative calculation is valid and the EWPT is strongly first order. In this region, when $X$ is an electrically charged color triplet, the phenomenology consists of an enhanced rate of $gg \to H$ and a reduced $H \to \gamma \gamma$ rate.
• Figure 3: An illustration of the behavior of $V_{\rm eff}$ as the limits $\lambda m^2 / 2 \mathcal{E}^2 \to 1$ (left) and $\lambda \to 0$ (right) are taken. The former leads to an EdSP whereas the latter leads to an EcSP.
• Figure 4: SFOPT correlated with large invisible decay in a neighborhood of the EcSP. The dashed lines corresponds to values of the singlet mass $m_S$. Left. The EW order parameter $v(T_c) / T_c$, for which $\mathrm{Eq.}\;(\ref{['eq:vcoverTc_Z2xSM']})$ is the leading-order expression. Right. The branching fraction of Higgs to an invisible S-pair ${\rm BR}_{\rm inv}$, for which the width $\mathrm{Eq.}\;(\ref{['eq:Z2xSM_Gammainv']})$ is the leading-order expression (see also Barger:2008jx).
• Figure 5: The EW order parameter $v(T_c) / T_c$ (blue) and invisible branching fraction ${\rm BR}_{\rm inv}$ (red), calculated as in Fig. \ref{['fig:EcSP_neighborhood']} but at the $\overline{\rm EcSP'}$ parameter point $\mathrm{Eq.}\;(\ref{['eq:Z2xSM_deltaEcSPbar']})$. As $a_2 / 2 \lambda$ is decreased below about $0.15$, Higgs invisible decay becomes sufficiently suppressed to evade collider constraints which impose ${\rm BR}_{\rm inv} \lesssim 0.64$. This threshold corresponds to $a_2 \approx 0.043$. At the same time, the electroweak phase transition remains strongly first order $v(T_c) / T_c > 1$.