On a Light Spinless Particle Coupled to Photons

A pseudoscalar or scalar particle $\varphi$ 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 $a\gamma \gamma$ coupling. We examine the laboratory, astrophysical and cosmological constraints on $\varphi$ and study whether it may constitute a substantial part of the dark matter. We also generalize the $\varphi$ interactions to possess $SU(2)\times U(1)$ gauge invariance, and analyze the phenomenological implications.