Production of Gravitational Waves in the nMSSM

TL;DR

This work evaluates gravitational-wave production from strongly first-order electroweak phase transitions in the nMSSM and a Standard Model extended with dimension-six Higgs operators. By solving for full multi-field bubble configurations, it derives the transition parameters that govern GW spectra from bubble collisions and turbulence, and assesses detectability with LISA and BBO. The main finding is that, across plausible parameter spaces, the GW relic density is generally below LISA sensitivity and often below BBO, due to transitions occurring at lower temperatures and producing larger bubbles with peak frequencies outside optimal detector ranges. The study also discusses the relationship between GW signals and electroweak baryogenesis, noting that observable GWs do not guarantee successful baryogenesis and that supersonic wall regimes can suppress diffusion-based BAU production, with substantial turbulence-model uncertainties affecting the predictions.

Abstract

During a strongly first-order phase transition gravitational waves are produced by bubble collisions and turbulent plasma motion. We analyze the relevant characteristics of the electroweak phase transition in the nMSSM to determine the generated gravitational wave signal. Additionally, we comment on correlations between the production of gravitational waves and baryogenesis. We conclude that the gravitational wave relic density in this model is generically too small to be detected in the near future by the LISA experiment. We also consider the case of a "Standard Model" with dimension-six Higgs potential, which leads to a slightly stronger signal of gravitational waves.

Figures (7)

• Figure 1: Different characteristics of the phase transition as functions of the parameter $M$. The Higgs mass is chosen to be $m_h = 120$ GeV.
• Figure 2: The spectrum of the density of GWs for different values of the parameter $M$. The used parameters are given in Tab. \ref{['finaltab_phi6']}. Smaller values of $M$ lead hereby to a stronger phase transition and larger GW production. In the shaded region, the sensitivity of LISA and BBO is expected to drop considerably. In the left (right) panel, the turbulent contribution to the GW spectrum from Ref. Dolgov:2002ra (Caprini:2006jb) has been used.
• Figure 3: The left panel shows the correlation between the parameters $\alpha$ and the typical size of the bubbles at the end of the phase transition, $\left< R \right>$. The right panel compares the typical bubble size with $\beta$ at first nucleation, $T=T_n$, and the end of the phase transition, $T=T_f$.
• Figure 4: The correlation between the parameter $\alpha$ and several properties of the phase transition: The temperature at first nucleation $T_n$, the temperature at the end of the phase transition $T_f$, the thickness of the bubble wall $l_w$ and the ratio between the Higgs vev and the temperature $\phi(T_f)/T_f$.
• Figure 5: The magnitude of the produced gravitational waves for our set of models. The panels show the GWs from collisions and from turbulence following the two different approaches under consideration.