Freezing-in the Pure Dark Axion Portal

TL;DR

This paper investigates the freeze-in production of a dark matter candidate in a dark sector with an axion-like particle and dark photon, considering different cosmological regimes and potential experimental probes.

Contribution

It introduces a novel analysis of dark matter production via a dark photon-axion portal, including cosmological constraints and experimental prospects.

Findings

Viable dark matter parameter space identified.

Potential detection at B-factories discussed.

Conditions for neglecting misalignment contribution outlined.

Abstract

A $Z_2$ symmetry under which "dark'' and "visible'' fields transform differently can further seclude already dark particles. In a dark sector consisting of an axion-like particle and a dark photon the dominant interaction can then be via a dark photon-axion portal to the ordinary photon. If the axion is lighter than the dark photon it naturally emerges as a viable dark matter candidate. We explore its freeze-in production both in a standard high reheating temperature and weak coupling as well as a low reheating temperature and strong coupling regime. Cosmological constraints are taken into account to identify viable regions in parameter space. For the strong freeze-in regime, we highlight the possibility of probing it at B-factories. We also briefly discuss a potential misalignment contribution to the dark matter density and delineate under which conditions it can be neglected.