Gravitational wave and collider implications of electroweak baryogenesis aided by non-standard cosmology

TL;DR

This work examines electroweak baryogenesis in a Standard Model extended by a Higgs-portal scalar and confronts it with a non-standard cosmology where an extra fast-redshifting component alters the early expansion rate. By computing the finite-temperature effective potential, bubble nucleation dynamics via $S_3$, and the sphaleron decoupling condition under modified $H$, the authors map viable EWBG parameter spaces for both neutral and colored scalar scenarios. They find that collider Higgs precision measurements, particularly at the ILC and future high-luminosity programs, provide robust probes that often outperform gravitational-wave searches, while a modified cosmology can broaden EWBG viability and suppress GW signals. The results emphasize that observing SM deviations in Higgs observables without corresponding GW signals would point toward non-standard cosmologies enabling EWBG, guiding experimental strategies across colliders and GW detectors.

Abstract

We consider various models realizing baryogenesis during the electroweak phase transition (EWBG). Our focus is their possible detection in future collider experiments and possible observation of gravitational waves emitted during the phase transition. We also discuss the possibility of a non-standard cosmological history which can facilitate EWBG. We show how acceptable parameter space can be extended due to such a modification and conclude that next generation precision experiments such as the ILC will be able to confirm or falsify many models realizing EWBG. We also show that, in general, collider searches are a more powerful probe than gravitational wave searches. However, observation of a deviation from the SM without any hints of gravitational waves can point to models with modified cosmological history that generically enable EWBG with weaker phase transition and thus, smaller GW signals.

Figures (7)

• Figure 1: Left panel: maximal modification of the Hubble parameter calculated at the nucleation temperatures $T_*=100$ GeV and $T_*=150$ GeV, as a function of the parameter $n$ which determines our cosmological model. Right panel: the modified required value of the ratio $v/T$ (evaluated at $T_*$) needed to preserve the baryon asymmetry created during the transition. This modified value from cosmological freeze-out of the sphaleron processes and is a function of the expansion rate normalized to the standard case of radiation dominated expansion.
• Figure 2: Region in parameter space of the neutral scalar mass $m_S$ and the Higgs mixing $g_S$ predicting successful baryogenesis together with predicted ILC experimental constraints on the triple Higgs coupling $\lambda_3$. Three different allowed (blue) regions correspond to standard cosmological history and expansion during the phase transition accelerated $10^3$ times and $10^6$ times.
• Figure 3: Values of neutral scalar mixing with the Higgs $g_S$ allowing successful baryogenesis due to modified cosmological history (light blue region) together with modification of the triple Higgs coupling $\lambda_3$. The dark blue region, and the region to the right with even larger mixing, predicts a strong enough phase transition without cosmological modification.
• Figure 4: Region in parameter space of new scalars mass $m_S$ and the Higgs mixing $g_S$ predicting successful baryogenesis together with modification of Higgs boson signal strength in $\gamma \gamma$ and $W W$ channels and modification of the triple Higgs coupling $\lambda_3$. Three different allowed (blue) regions correspond to standard cosmological history and expansion during the phase transition accelerated $10^3$ times and $10^6$ times.
• Figure 5: Values of the Higgs mixing $g_S$ allowing successful baryogenesis due to modified cosmological history (light blue region) together with modification in Higgs boson signals in the right handed stop model. The dark blue region (and even larger mixing region) predicts a strong enough phase transition without cosmological modification.