Electroweak Baryogenesis and Higgs Signatures

TL;DR

This work investigates whether a strongly first-order electroweak phase transition (EWPT) compatible with electroweak baryogenesis (EWBG) can occur in extensions of the SM that couple a new colored scalar X to the Higgs via a Higgs portal. The authors show that such a colored scalar can both strengthen the EWPT (raising $\phi_C/T_C$) and modify loop-induced Higgs couplings to gluons and photons, leading to observable shifts in gluon-fusion production and $h\to\gamma\gamma$ decays, especially for $m_h$ in the 115–130 GeV range. Their results indicate that in the viable EWBG parameter space, the Higgs production rate via gluon fusion is enhanced by at least $\sim 1.6$, with additional dependence on the number of triplets $N$ and the electric charge $q_X$, while the diphoton channel exhibits a more modest change due to interference with the $W$ loop. Applying the framework to the MSSM, they find substantial tension: a SM-like Higgs at $m_h\approx125$ GeV would rule out the MSSM EWBG window, whereas a strongly modified Higgs signal would support EWBG in this class of models. Overall, the paper establishes a concrete, testable link between EWBG viability and Higgs couplings that can be probed with current and future LHC data, including direct X searches and precision Higgs measurements.

Abstract

We explore the connection between the strength of the electroweak phase transition and the properties of the Higgs boson. Our interest is in regions of parameter space that can realize electroweak baryogenesis. We do so in a simplified framework in which a single Higgs field couples to new scalar fields charged under SU(3)_c by way of the Higgs portal. Such new scalars can make the electroweak phase transition more strongly first-order, while contributing to the effective Higgs boson couplings to gluons and photons through loop effects. For Higgs boson masses in the range 115 GeV < m_h < 130 GeV, whenever the phase transition becomes strong enough for successful electroweak baryogenesis, we find that Higgs boson properties are modified by an amount observable by the LHC. We also discuss the baryogenesis window of the minimal supersymmetric standard model (MSSM), which appears to be under tension. Furthermore, we argue that the discovery of a Higgs boson with standard model-like couplings to gluons and photons will rule out electroweak baryogenesis in the MSSM.

Figures (4)

• Figure 1: Contours of $\phi_C/T_C$ [black, solid lines] in the $-\mathrm{sgn}\left(M_X^2\right) \sqrt{|M_X^2|}$ vs. $Q$ plane for one new color-triplet scalar. (The most negative mass squared values are at the top of the plot.) The bold line corresponds to $\phi_C/T_C = 0.9$, and the adjacent solid lines delineate steps of $\Delta(\phi_C/T_C) = 0.2$. The yellow shaded region is excluded because for these parameters, the Universe would have evolved to a charge-color breaking minimum. In the left plot, we also show contours of the ratio of the gluon fusion cross section to the SM value [red, dotted lines]. In the right plot, we show contours of the ratio of the gluon fusion cross section times the branching ratio to di-photons to the SM value [red, dotted lines] when the charge of the colored scalar is taken to be $q_X=2/3$.
• Figure 2: Contours of $\phi_C/T_C$ [black, solid lines] and $\sigma\times \mathrm{BR}$ [red, dotted lines] in the $-\mathrm{sgn}\left(M_X^2\right) \sqrt{|M_X^2|}$ vs. $Q$ plane for one new color-triplet scalar. In the left plot, we have taken $q_X = 1/3$ and in the right, we have $q_X = 4/3$. The yellow region shows the range of parameters for which the Universe would have evolved to a charge-color breaking vacuum. For details, see Fig. \ref{['fig:sigmaWithPT_N1']}.
• Figure 3: Contours of $\phi_C/T_C$ [black, solid lines] and $\sigma$ (left) or $\sigma\times \mathrm{BR}$ (right) [red, dotted lines] in the $-\mathrm{sgn}\left(M_X^2\right) \sqrt{|M_X^2|}$ vs. $Q$ plane for two new color-triplet scalars with $q_X=2/3$. The yellow region shows the range of parameters for which the Universe would have evolved to a charge-color breaking vacuum. For details, see Fig. \ref{['fig:sigmaWithPT_N1']}.
• Figure 4: Contours of $\phi_C/T_C$ [black, solid lines] and $\sigma\times \mathrm{BR}$ [red, dotted lines] in the $-\mathrm{sgn}\left(M_X^2\right) \sqrt{|M_X^2|}$ vs. $Q$ plane for the MSSM-like model. On the left (right) we have taken the Higgs boson mass to be 115 GeV (125 GeV). The yellow region shows the range of parameters for which the Universe would have evolved to a charge-color breaking vacuum. For details, see Fig. \ref{['fig:sigmaWithPT_N1']}.