Dark matter in QCD-like theories with a theta vacuum: cosmological and astrophysical implications

TL;DR

This paper explores how a non-zero theta vacuum angle in QCD-like theories can lead to novel dark matter production mechanisms and self-interactions, with potential observable signals like gravitational waves.

Contribution

It introduces a new dark matter production mechanism via resonant processes triggered by the theta angle, and discusses velocity-dependent self-interactions without light mediators.

Findings

Resonant number-changing processes can produce MeV dark matter consistent with observations.

Specific meson spectra enable velocity-dependent self-interacting dark matter.

Potential gravitational wave signals from dark pion chiral phase transitions.

Abstract

QCD-like theories in which the dark matter (DM) of the Universe is hypothesized to be a thermal relic in the form of a dark pion has been extensively investigated, with most studies neglecting the CP-violating $\theta$-angle associated with the topological vacuum. We point out that a non-vanishing $\theta$ could potentially trigger resonant number-changing processes giving rise to the observed relic density in agreement with perturbative unitarity as well as observations of clusters of galaxies. This constitutes a novel production mechanism of MeV DM and an alternative to those relying on the Wess-Zumino-Witten term. Moreover, for specific meson mass spectra, similar resonant scatterings serve as a realization of velocity-dependent self-interacting DM without a light mediator. Explicit benchmark models are presented together with a discussion of possible signals, including gravitational waves from the chiral phase transition associated with the dark pions.