A Heavy QCD Axion and the Mirror World

TL;DR

This paper proposes a model with a heavy QCD axion connecting the Standard Model and a mirror sector, explaining dark matter, solving the strong CP problem, and predicting observable cosmological and particle physics signals.

Contribution

It introduces a heavy axion portal between sectors, linking dark matter, strong CP solution, and cosmological signals in a highly predictive framework.

Findings

Dark matter from mirror electrons of 225 GeV mass

Primordial gravitational waves from mirror QCD phase transition

Effects on large-scale structure and cosmic microwave background

Abstract

We study the mirror world with dark matter arising from the thermal freeze-out of the lightest, stable mirror particle -- the mirror electron. The dark matter abundance is achieved for mirror electrons of mass 225 GeV, fixing the mirror electroweak scale near $10^8$ GeV. This highly predictive scenario is realized by an axion that acts as a portal between the two sectors through its coupling to the QCD and mirror QCD sectors. The axion is more massive than the standard QCD axion due to additional contributions from mirror strong dynamics. Still, the strong CP problem is solved by this "heavy" axion due to the alignment of the QCD and mirror QCD potentials. Mirror entropy is transferred into the Standard Model sector via the axion portal, which alleviates overproduction of dark radiation from mirror glueball decays. This mirror scenario has a variety of signals: (1) primordial gravitational waves from the first-order mirror QCD phase transition occurring at a temperature near 35 GeV, (2) effects on large-scale structure from dark matter self-interactions from mirror QED, (3) dark radiation affecting the cosmic microwave background, and (4) the rare kaon decay, $K^+ \rightarrow (\pi^+ + \rm{axion})$. The first two signals do not depend on any fundamental free parameters of the theory while the latter two depend on a single free parameter, the axion decay constant.