Electroweak Phase Transition and Baryogenesis in the nMSSM

TL;DR

The paper investigates electroweak baryogenesis in the nMSSM with CP violation confined to the singlet sector. It shows that a strong first-order electroweak phase transition arises generically from tree-level singlet interactions, enabling viable baryogenesis through chargino-mediated transport without requiring a light stop. Using a basis-independent, gradient-expansion transport framework, the authors compute the bubble-wall profiles and CP-violating sources, finding that the dominant contribution often comes from a second-order semiclassical force, with the BAU scaling roughly as $\eta \propto \frac{\Delta q_s}{l_w\,T_c}$ and can reach the observed value for reasonable parameter choices. EDM constraints imply heavy first- and second-generation sfermions and often a small CP-odd phase in the broken phase, but cancellations or modest CP violation allow compatibility with data; overall, EWBG in the nMSSM emerges as a robust and testable alternative to MSSM scenarios.

Abstract

We analyze the nMSSM with CP violation in the singlet sector. We study the static and dynamical properties of the electroweak phase transition. We conclude that electroweak baryogenesis in this model is generic in the sense that if the present limits on the mass spectrum are applied, no severe additional tuning is required to obtain a strong first-order phase transition and to generate a sufficient baryon asymmetry. For this we determine the shape of the nucleating bubbles, including the profiles of CP-violating phases. The baryon asymmetry is calculated using the advanced transport theory to first and second order in gradient expansion presented recently. Still, first and second generation sfermions must be heavy to avoid large electric dipole moments.

Figures (9)

• Figure 1: The dependence of $\eta_{10} \equiv 10^{10}\eta$ on the chargino mass parameter $\Delta\mu=\mu_0$. The parameters used are $l_w = 10/T_c$ and $\Delta q_s=\pi/10$.
• Figure 2: A typical wall profile for the parameter $q_s$, corresponding to the parameters $\Delta q_s=0.119$ and $l_w = 4.81\, T_c^{-1}$.
• Figure 3: The critical temperature $T_c$, the Higgs vev $\phi$ in the broken phase at $T_c$ and one Higgs mass as functions of $a_\lambda$.
• Figure 4: The parameters $\lambda$ and $q_t$ for a set of random models that fulfill the mass constraints.
• Figure 5: The plots show the combinations of the parameters that enter the tree-level condition for a first-order phase transition, Eq. (\ref{['pt_con']}). The left plot contains parameter sets that fulfill only the mass constraints, while the right plot contains parameter sets that have in addition a strong first-order phase transition.