On a Light Spinless Particle Coupled to Photons

TL;DR

The paper investigates a light spinless φ that couples exclusively to photons through a two-photon vertex, exploring whether such a particle could be detected in axion-search experiments and possibly serve as dark matter. It systematically derives constraints from laboratory (non-dedicated and dedicated) experiments, astrophysical energy-loss arguments, and cosmological relic abundance considerations, including a gauge-invariant extension that introduces a γZφ coupling. The findings show φ is typically a hot relic and cannot easily account for cold dark matter, with strong bounds across most mass ranges and particularly stringent limits from laser/solar searches and LEP-like γZφ interactions. Consequently, the allowed φ parameter space is highly constrained, with only narrow, challenging-to-probe windows around certain eV–keV masses remaining for dark-matter-related scenarios.

Abstract

A pseudoscalar or scalar particle $φ$ that couples to two photons but not to leptons, quarks and nucleons would have effects in most of the experiments searching for axions, since these are based on the $a γγ$ coupling. We examine the laboratory, astrophysical and cosmological constraints on $φ$ and study whether it may constitute a substantial part of the dark matter. We also generalize the $φ$ interactions to possess $SU(2) \times U(1)$ gauge invariance, and analyze the phenomenological implications.