On the wondrous stability of ALP dark matter

Abstract

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