Enhancement of second-order gravitational waves at Q-ball decay

TL;DR

This paper explores how Q-ball decay after the electroweak phase transition can generate large lepton asymmetry and enhance second-order gravitational waves, which may be detectable by pulsar timing arrays.

Contribution

It demonstrates that Q-ball decay can produce observable second-order gravitational waves, linking early universe particle physics with gravitational wave detection.

Findings

Q-ball decay induces a sharp transition from matter to radiation domination.

Enhanced second-order gravitational waves can be probed by pulsar timing arrays.

The scenario explains large lepton asymmetry consistent with helium observations.

Abstract

The recent observation of $^4$He favors a large lepton asymmetry at the big bang nucleosynthesis. If Q-balls with a lepton charge decay after the electroweak phase transition, such a large lepton asymmetry can be generated without producing too large baryon asymmetry. In this scenario, Q-balls dominate the universe before the decay and induces the sharp transition from the early matter-dominated era to the radiation-dominated era. In this transition, the gravitational waves (GWs) are enhanced through a second-order effect of the scalar perturbations. We evaluate the density of the produced GWs and show that pulsar timing array observations can probe this scenario depending on the amplitude of the scalar perturbations.