On the Wondrous Stability of ALP Dark Matter

TL;DR

This paper investigates the stability of axion-like particles (ALPs) as dark matter, analyzing how cosmological effects like expansion and plasma mass influence their decay rates, especially considering Bose enhancement.

Contribution

It provides a detailed review of ALP decay mechanisms including Bose enhancement and demonstrates stability conditions involving cosmic expansion and plasma effects.

Findings

Bose enhancement can potentially accelerate ALP decay.

Cosmic expansion and plasma mass effects can prevent rapid decay.

Stability of ALPs depends on a combination of these cosmological factors.

Abstract

The very low mass and small coupling of axion-like particles (ALPs) is usually taken as a guarantor of their cosmological longevity, making them excellent dark matter candidates. That said, Bose enhancement could stimulate decays and challenge this paradigm. Here, we analyze and review the cosmological decay of ALPs into photons, taking Bose enhancement into account, thereby going beyond the usual naive perturbative estimate. At first glance, this calculation seems to yield an exponentially growing resonance and therefore an extremely fast decay rate. However, the redshifting of the decay products due to the expansion of the Universe as well as the effective plasma mass of the photon can prevent an efficient resonance. While this result agrees with existing analyses of the QCD axion, for more general ALPs that can feature an enhanced photon coupling, stability is only ensured by a combination of the expansion and the plasma effects.