Opening the Window for Electroweak Baryogenesis

TL;DR

The paper investigates electroweak baryogenesis within the MSSM by exploiting a light-stop sector to strengthen the finite-temperature phase transition, potentially achieving $v(T_c)/T_c \gtrsim 1$ while respecting current Higgs and sparticle bounds. It analyzes color-breaking minima at zero and finite temperature to derive viable bounds on the stop-sector parameters and to understand how stop mixing affects the phase transition. The authors present quantitative results showing a strong transition (up to $v(T_c)/T_c \approx 1.75$) for realistic spectra and discuss how precision electroweak constraints shape the allowed region, including implications for low $\tan\beta$ and light Higgs masses. They conclude that MSSM electroweak baryogenesis remains viable in substantial parameter regions, though non-perturbative checks and higher-loop corrections are needed for robust confirmation.

Abstract

We perform an analysis of the behaviour of the electroweak phase transition in the Minimal Supersymmetric Standard Model, in the presence of light stops. We show that, in previously unexplored regions of parameter space, the order parameter $v(T_c)/T_c$ can become significantly larger than one, for values of the Higgs and supersymmetric particle masses consistent with the present experimental bounds. This implies that baryon number can be efficiently generated at the electroweak phase transition. As a by-product of this study, we present an analysis of the problem of colour breaking minima at zero and finite temperature, and we use it to investigate the region of parameter space preferred by the best fit to the present precision electroweak measurement data, in which the left-handed stops are much heavier than the right-handed ones.

Figures (4)

• Figure 1: Plot of $V_{\rm min}/|V_{EW}|$ for $m_t=175$ GeV, $m_Q=500$ GeV, $\tan\beta=1.7$ and $\widetilde{A}_t$=430 GeV [upper curve]--444 GeV [lower curve], step=2 GeV.
• Figure 2: Plot of $v(T_c)/T_c$ as a function of $m_{\,\widetilde{t}}$ for $m_Q$ and $m_t$ as in Fig. 1, $\widetilde{A}_t=0$ and $\tan\beta=2$. The diamond [cross, star] denotes the value of $\widetilde{m}_U$ for which the bound, Eq. (3.4) [Eq. (4.5) with $E_U$ given by Eq. (4.3), Eq. (4.5) with $E_U = E_U^g$] is saturated.
• Figure 3: Plot of $v(T_c)/T_c$ as functions of $\tan\beta$ for $m_Q$ and $\widetilde{A}_t$ as in Fig. 2, and $m_U$ saturating Eq. (3.4) [solid] and Eq. (4.5) [thick dashed line for $E_U$ given by Eq. (4.3) and thin dashed line for $E_U = E_U^g$]. The additional thin lines are plots of $m_H$ in units of 65 GeV [solid] and $m_{\,\widetilde{t}}$ in units of $m_t$ [short-dashed], corresponding to the values of $\widetilde{m}_U$ associated with the solid line.
• Figure 4: The same as in Fig. 3, but as functions of $\widetilde{A}_t$, for $\tan\beta=1.7$.