String-mediated electroweak baryogenesis: a critical analysis

TL;DR

This work assesses whether electroweak baryogenesis can be mediated by nonsuperconducting cosmic strings. It computes the sphaleron rate in symmetry-restored string cores and analyzes CP-violating sources and diffusion of asymmetries within string walls, using both model-independent bounds and concrete two-Higgs-doublet models. The key finding is that, for thin or typical non-integer flux strings, the sphaleron rate is not sufficiently unsuppressed, and overall baryon production from a string network is far below the observed abundance, by at least 10–13 orders of magnitude under optimistic assumptions. The results strongly challenge string-mediated electroweak baryogenesis in non-superconducting scenarios, suggesting that only superconducting strings with unrealistically wide restoration regions could potentially approach viability.

Abstract

We study the scenario of electroweak baryogenesis mediated by nonsuperconducting cosmic strings. This idea relies upon electroweak symmetry being restored in a region around the core of the topological defect so that, within this region, the rate of baryon number violation is enhanced. We compute numerically how effectively baryon number is violated along a cosmic string, at an epoch when the baryon number violation rate elsewhere is negligible. We show that B-violation along nonsuperconducting strings is quite inefficient. When proper accounting is taken of the velocity dependence of the baryon number production by strings, it proves too small to explain the observed abundance by at least ten orders of magnitude, whether the strings are in the friction dominated or the scaling regime.

Figures (8)

• Figure 1: Higgs and gauge field profile functions for the string and the sphaleron. The Higgs field rises faster in the string, but the $Z$ field rises more slowly than the gauge field of the sphaleron. In each case, $\lambda = g^2 / 8$.
• Figure 2: Doublet and singlet Higgs field profiles around a string, for the parameters $\lambda = 0.35$, $\lambda_s = 8.8\times 10^{-5}$, $\gamma = 9.4\times 10^{-3}$, and $S_0 = 63 \phi_0$, where $\Phi_0 = 174$ GeV. These parameters do not allow thermal symmetry restoration of the $S$ field.
• Figure 3: (a,b) Dependence of the Higgs field central value and half-width, $\phi(0)$, $r_{1/2}$ respectively, on the potential parameters $x = \sqrt{\lambda/\lambda_s}$ and $y = S_0/\Phi_0$, for $\hat{\gamma} = 1.7$ and $z=1$. The logarithm is base 10. (c) Contours of the dimensionless Higgs mass $\hat{m}_h^2 = m_h^2/\Phi_0^2$ for the same parameters.
• Figure 4: Same as figure 3 but with $\hat{\gamma} = 0.1$
• Figure 5: The chiral asymmetry as a function of distance from the center of the string, for the parameters $D=6/T$, $\lambda = D$, $\Gamma = T/100$, $w=6/T$, $\delta_p = 0.1 T$, and $v = 0.1$.