Lepton-mediated electroweak baryogenesis

TL;DR

The paper shows that tau and bottom Yukawa interactions during the electroweak phase transition can be fast enough to affect transport and generate a lepton-dominated left-handed charge $n_L$, leading to a lepton-mediated electroweak baryogenesis (EWB) scenario in the MSSM. It derives a comprehensive set of Boltzmann equations including $\Gamma_{yb}$ and $\Gamma_{y\tau}$, provides analytic expressions for $n_L$ in terms of the Higgs density $H$, and solves a master equation for $H$ to obtain the baryon asymmetry $n_B/s$. The authors numerically validate the analytic results, compute the $b$ and $\tau$ Yukawa rates, and demonstrate that with Yukawas in equilibrium the BAU is generated predominantly from leptons and can flip sign compared to neglecting these rates. They identify parameter regions (e.g., large $\tan\beta$, light Higgs sector, near-degenerate right-handed squarks) where the leptonic contribution dominates and discuss the implications for testing SUSY EWB with LHC data and EDM searches.

Abstract

We investigate the impact of the tau and bottom Yukawa couplings on the transport dynamics for electroweak baryogenesis in supersymmetric extensions of the Standard Model. Although it has generally been assumed in the literature that all Yukawa interactions except those involving the top quark are negligible, we find that the tau and bottom Yukawa interaction rates are too fast to be neglected. We identify an illustrative "lepton-mediated electroweak baryogenesis" scenario in which the baryon asymmetry is induced mainly through the presence of a left-handed leptonic charge. We derive analytic formulae for the computation of the baryon asymmetry that, in light of these effects, are qualitatively different from those in the established literature. In this scenario, for fixed CP-violating phases, the baryon asymmetry has opposite sign compared to that calculated using established formulae.

Figures (3)

• Figure 1: Examples of bottom and tau Yukawa interactions from (a) absorption/decay, and (b) scattering processes involving an addition gauge boson, showing how Higgs density is converted into left-handed quark and lepton density. Third generation Yukawa couplings also give rise to F-term-induced scattering processes (c).
• Figure 2: Contour plot of $\tau_{\rm diff}/\tau_{yb}$ (left) and $\tau_{\rm diff}/\tau_{y\tau}$ (right) in $\tan\beta$-$m_A$ parameter space. Values of $\tau_{yb}$ and $\tau_{y\tau}$ include contributions from $H_d \leftrightarrow q_L b_R$ and $H_d \leftrightarrow \ell_L \tau_R$ only. Tau Yukawa chemical equilibrium is maintained for $\tau_{\rm diff}/\tau_{y\tau} \gtrsim 10$; similarly for the bottom Yukawa.
• Figure 3: Left-handed charge densities for leptons (left panel) and quarks (right panel) that generate $n_B/s$, for lepton-mediated scenario. Solid (dashed) curves are numerical (analytic) results, as function of distance $z$ from bubble wall. Shaded region denotes broken electroweak symmetry. Dotted curves are numerical results obtained neglecting tau/bottom Yukawa interactions. The effect of these interactions is to suppress LH quark charge, while enhancing LH lepton charge, thereby flipping the sign of $n_L$ and $n_B/s$ compared to previous computations.