Do direct detection experiments constrain axionlike particles coupled to electrons?

TL;DR

Laboratory and astrophysical constraints on axionlike particles with electron couplings show that many previously considered limits are invalid because these ALPs decay too quickly to be dark matter, with gamma-ray and X-ray data providing stronger bounds.

Contribution

This paper demonstrates that many experimental limits on ALPs as dark matter are invalid due to their decay, and highlights the stronger astrophysical constraints in the keV-MeV mass range.

Findings

Many ALP parameter spaces are excluded due to rapid decay.

Gamma-ray and X-ray observations set stronger bounds than laboratory experiments.

Most constraints are model-independent and difficult to evade without fine-tuning.

Abstract

Several laboratory experiments have published limits on axionlike particles (ALPs) with feeble couplings to electrons and masses in the keV-MeV range, under the assumption that such ALPs comprise the dark matter. We note that ALPs decay radiatively into photons, and show that for a large subset of the parameter space ostensibly probed by these experiments, the lifetime of the ALPs is shorter than the age of the universe. Such ALPs cannot consistently make up the dark matter, which significantly affects the interpretation of published limits from GERDA, Edelweiss-III, SuperCDMS and Majorana. Moreover, constraints from gamma-ray and X-ray astronomy exclude an even wider range of the ALP-electron coupling, and supersede all current experimental limits on dark matter ALPs in the 6 keV to 1 MeV mass range. These conclusions are rather model-independent, and can only be avoided at the expense of significant fine-tuning in theories where the ALP has additional couplings to other particles.