Bubble wall velocity for first-order QCD phase transition

TL;DR

This paper estimates the bubble wall velocity during a first-order QCD phase transition influenced by nonstandard effects, finding it lower than previously assumed, which suppresses gravitational wave signals near experimental detection thresholds.

Contribution

It provides a simple estimate of bubble wall velocity in the small supercooling regime, incorporating recent progress and robust against detailed QCD equation of state data.

Findings

Wall velocity is significantly lower than previous estimates.

Gravitational wave production is suppressed, near but below detection thresholds.

Results remain robust with advanced QCD equation of state data.

Abstract

Although the QCD phase transition is a crossover in the standard model, nonstandard effects such as a large lepton asymmetry are known to make it first order, with possible applications to gravitational wave production. This process is sensitive to the speed of the bubble walls during the phase transition, which is difficult to compute from first principles. We take advantage of recent progress on wall speed determinations to provide a simple estimate valid in the small supercooling regime which constrains the wall speed to be significantly lower than what has been used in previous literature. This in turn strongly suppresses the production of gravitational waves, to a level that is just out of reach of the most sensitive projected experiment for this signal, $\mu$Ares. While our analysis approximates the equation of state using the template model, we demonstrate that our conclusions remain robust when incorporating state-of-the-art QCD equation of state data.