Inverse Electroweak Baryogenesis

Abstract

We propose a mechanism for baryogenesis in which the baryon asymmetry is generated as an \emph{equilibrium response} of weak sphalerons in a region where electroweak sphaleron transitions remain unsuppressed, $h/T\lesssim 1$. A nonzero equilibrium baryon density arises in the presence of an approximately conserved global charge $X$, carried by states with nonzero hypercharge and, after electroweak symmetry breaking, electric charge. Plasma screening enforces gauge-charge neutrality, so an $X$ asymmetry induces compensating gauge-charge densities in the Standard Model plasma, which in turn bias weak sphaleron transitions toward a state with nonvanishing baryon number. The required $X$ asymmetry is generated during a phase transition that changes the strength of electroweak symmetry breaking, but need not coincide with the final electroweak phase transition. In particular, the mechanism can operate during an inverse electroweak phase transition, where baryon number is produced behind the advancing wall, in contrast to conventional electroweak baryogenesis. Because baryon production is decoupled from a direct first-order electroweak phase transition, the scenario can be realized at parametrically higher temperatures than standard electroweak baryogenesis, thereby weakening current experimental constraints. This framework provides a qualitatively distinct route to electroweak baryogenesis, with different parametric dependence, phase-transition dynamics, and phenomenological signatures.

Figures (5)

• Figure 1: Schematic illustration of the mechanism of baryon-number generation during an inverse electroweak phase transition. CP-violating interactions with the phase boundary, together with $X$-violating processes in front of the wall, generate a static excess of an approximately conserved global charge $X$ in the region behind the wall. Since the $X$-charge carriers also carry hypercharge, overall gauge-charge neutrality (enforced by plasma screening) requires compensating hypercharge distributed among the Standard Model species. The resulting nonzero hypercharge of SM induces a nonvanishing equilibrium baryon number once weak sphaleron processes become active.
• Figure 2: Upper plot: Time evolution of some of the chemical potentials, and the combined left-handed chemical potential $\mu_L$ (in black) for a unit initial $\mu_{\tau_R}/T$. Lower plot: Time evolution of the baryon number density for the same case. Gray vertical lines labeled with $\Gamma_{\text{ws}}$ and $\Gamma_{y_\tau}$ show the times at which weak sphalerons and tau-Yukawa-mediated interactions equilibrate. Blue dashed line corresponds to the evolution with all lepton Yukawas set to zero, demonstrating the effect of $n_{l_R}$ charges if they were exact.
• Figure 3: Same as in Fig.\ref{['fig:evolnum1']} but for the unit initial $\mu_X/T = (\mu_{\Phi_+}+\mu_{\Phi_0})/T$ and vanishing $\mu_{\tau_R}/T$. Gray horizontal lines in the lower plot correspond to the equilibrium baryon number values reached upon equilibration of corresponding Yukawa interactions.
• Figure 4: Sketch of the transport system \ref{['eq:toytransport']} solution for the chemical potential $\mu$ and the velocity perturbation $u$ across the phase transition wall, in the wall frame, with a point-like source at $z=0$, and assuming charge conservation behind the wall at $z<0$. $\vec{v}_{\text{wall}}$ shows the wall speed direction in the plasma frame. The forward diffusing part decays with a characteristic length $1/\lambda_-$ in front of the wall. The solution also demonstrates the trail of static ($u=0$) charge distribution left behind the source.
• Figure 5: Chemical potentials in the vicinity of the inverse electroweak wall. The wall is centered around $z=0$. The black curve shows the (multiplied by 10) combination $\mu_X=\mu_{\Phi_+}+\mu_{\Phi_0}$, that is conserved behind the wall, and determines the baryon asymmetry generated later through Eq. \ref{['eq:nBXQ']}.