Baryogenesis and first-order QCD transition with gravitational waves from a large lepton asymmetry

TL;DR

This paper explores how a large primordial lepton asymmetry can induce a first-order QCD transition and generate detectable gravitational waves, with implications for baryogenesis and future GW observations.

Contribution

It refines the required lepton asymmetry for a first-order QCD transition using advanced theoretical methods and links it to potential gravitational wave signals.

Findings

Lepton asymmetry needed is an order of magnitude smaller than previous estimates.

A first-order QCD transition can occur without entropy dilution.

Upcoming GW experiments could detect signals related to sphaleron freeze-in.

Abstract

A large primordial lepton asymmetry can lead to successful baryogenesis by preventing the restoration of electroweak symmetry at high temperatures, thereby suppressing the sphaleron rate. This asymmetry can also lead to a first-order cosmic QCD transition, accompanied by detectable gravitational wave (GW) signals. By employing next-to-leading order dimensional reduction we determine that the necessary lepton asymmetry is approximately one order of magnitude smaller than previously estimated. Incorporating an updated QCD equation of state that harmonizes lattice and functional QCD outcomes, we pinpoint the range of lepton flavor asymmetries capable of inducing a first-order cosmic QCD transition. To maintain consistency with observational constraints from the Cosmic Microwave Background and Big Bang Nucleosynthesis, achieving the correct baryon asymmetry requires entropy dilution by approximately a factor of ten. However, the first-order QCD transition itself can occur independently of entropy dilution. We propose that the sphaleron freeze-in mechanism can be investigated through forthcoming GW experiments such as $\mu$Ares.