Electroweak Baryogenesis and Higgs and Stop Searches at LEP and the Tevatron

TL;DR

The paper investigates electroweak baryogenesis within the MSSM by requiring a strongly first-order phase transition, achievable if the Higgs is light and the light stop is sufficiently light. It performs a detailed two-loop finite-temperature analysis of the Higgs and stop sectors, including stop mixing and color-breaking directions, to map the allowed region in the $(m_h, m_{ ilde{t}})$ plane and assess vacuum stability. The study finds that, under absolute vacuum stability, $m_h$ must lie roughly between $75$ and $105$ GeV with $m_{ ilde{t}}$ in the vicinity of or below $m_t$, though metastability or two-step transitions can enlarge this region; two-loop effects are crucial in determining these bounds. The results motivate extensive Higgs and stop searches at LEP and the Tevatron and call for non-perturbative validation of the perturbative analysis to firmly establish the viability of electroweak baryogenesis in this framework.

Abstract

It has been recently shown that the observed baryon number may originate at the electroweak phase transition, provided that the Higgs boson and the lightest stop are sufficiently light. In this work, we perform a detailed analysis, including all dominant two-loop finite temperature corrections to the Higgs effective potential, as well as the non-trivial effects proceeding from the mixing in the stop sector, to define the region of parameter space for which electroweak baryogenesis can happen. The limits on the stop and Higgs masses are obtained by taking into account the experimental bounds on these quantities, as well as those coming from the requirement of avoiding dangerous color breaking minima. We find for the Higgs mass $m_h \simlt 105$ GeV, while the stop mass may be close to the present experimental bound and must be smaller than, or of order of, the top quark mass. These results provide a very strong motivation for further non-perturbative analysis of the electroweak phase transition, as well as for the search for Higgs and stop particles at the LEP and Tevatron colliders.

Figures (2)

• Figure 1: Values of $m_h$, $m_{\;\widetilde{t}}$ for which $v(T_c)/T_c = 1$ (solid line), $T_c^U = T_c$ (dashed line), $\widetilde{m}_U = \widetilde{m}_U^c$ (short-dashed line), for $m_Q = 1$ TeV and different fixed values of $\widetilde{A}_t$. The region on the left of the solid line is consistent with a strongly first order phase transition. A two step phase transition may occur in the regions on the left of the dashed line, while on the left of the short-dashed line, the physical vacuum at $T = 0$ becomes metastable. The region on the left of both the dashed and short-dashed lines leads to a stable color breaking vacuum state at zero temperature and is hence physically unacceptable.
• Figure 2: Region of the $m_h$--$m_{\;\widetilde{t}}$ parameter space for which a strongly first order phase transition takes place is shown within solid lines. The short-dashed lines demark the region for which a two-step phase transition may occur. The region on the right of the dashed line and left of the short-dashed may lead to a metastable vacuum state.