Baryogenesis and Gravitational Waves from Runaway Bubble Collisions

TL;DR

This work introduces a novel, testable mechanism for baryogenesis at temperatures well below the electroweak scale, exploiting a strong first-order phase transition in a hidden sector that drives runaway bubble walls. Collisions of these walls non-thermally produce heavy states that decay out of equilibrium via CP-violating, baryon-number-violating operators, yielding the observed baryon asymmetry without relying on high-temperature reheating. A concrete, perturbative hidden-sector model with an $SU(2)$ gauge group and a dark Higgs demonstrates the mechanism, discusses cosmological safety, SM couplings, and heavy-state production, and shows that the resulting baryon abundance can match observations for reasonable parameters. A key, model-independent prediction is a stochastic gravitational-wave background from the hidden PT, potentially detectable by eLISA, which offers a promising experimental window into this baryogenesis paradigm. Open questions include the precise dynamics of runaway bubbles, the range of viable Lorentz factors, and how broadly this mechanism can be embedded in different hidden-sector constructions.

Abstract

We propose a novel mechanism for production of baryonic asymmetry in the early Universe. The mechanism takes advantage of the strong first order phase transition that produces runaway bubbles in the hidden sector that propagate almost without friction with ultra-relativistic velocities. Collisions of such bubbles can non-thermally produce heavy particles that further decay out-of-equilibrium into the SM and produce the observed baryonic asymmetry. This process can proceed at the very low temperatures, providing a new mechanism of post-sphaleron baryogenesis. In this paper we present a fully calculable model which produces the baryonic asymmetry along these lines as well as evades all the existing cosmological constraints. We emphasize that the Gravitational Waves signal from the first order phase transition is completely generic and can potentially be detected by the future eLISA interferometer. We also discuss other potential signals, which are more model dependent, and point out the unresolved theoretical questions related to our proposal.

Figures (7)

• Figure 1: Runaway bubbles in potentials which satisfy Bödeker-Moore criterion. The red line stands for the full one-loop thermal potential, while the blue line stands for the mean free field approximation. If the potentials look like on the right panel, the bubbles during the first order phase transition clearly have runaway behavior. The picture on the left panel does not satisfy the criterion: although the local symmetry breaking minimum exists both in the full potential and in the mean free field approximation, the vacuum energy of the symmetry breaking minimum in the mean field approximation is bigger than the vacuum energy of the symmetry preserving phase.
• Figure 2: The upper bound on the nucleation temperature (red line) and the highest temperature at which BM criterion for the runaway bubbles is satisfied (blue line). Below the black line the barrier between the real vacuum and the false vacuum disappears. In the most pessimistic scenario the relevant parameter space lies to the right of the intersection between the blue and the red curves, while in the most optimistic scenario it is to the right of the intersection between the black and the blue curves.
• Figure 3: Constraints on the value of $\kappa$ in the higgs portal coupling for different values of $f$. Above the blue line the structure of the hidden sector potential may be altered without appropriate fine-tuning due to too large induced dark higgs mass. Below the purple line the decay time of the dark higgs exceeds 1 sec. The shaded region is allowed. The constraints from the invisible higgs decays $\kappa \lesssim 0.01$ are completely subdominant and they are not shown on these plots.
• Figure 4: CP violating decays of the heavy fermions. In order to produce the asymmetry, one needs the interference between the (first) tree level diagram and the loop level diagrams, which are sensitive to the irreducible phases of the couplings of the scalar $\Delta$. The blobs stand for the mixings between the Majorana particles, that we necessary have in the most generic model. These mixings are unavoidable because all the phases in the single-generation couplings are removable.
• Figure 5: Dark gauge boson decay at one-loop, induced by the couplings of the heavy fermions and the scalar $\Delta$ to the SM. The coupling to the fermions is gauge coupling suppressed, but assuming $\lambda' \sim1$ there are no suppressions in the Yukawa interactions.