Constraints on Scalar Coupling to Electromagnetism

We review a possible nonminimal (dilatonic) coupling of a scalar field (axion-like particle) to electromagnetism, through experimental and observational constraints. Such a coupling is motivated from recent quasar spectrum observations that indicate a possible spatial and/or temporal variation of the fine-structure constant. We consider a dilatonic coupling of the form BF (phi) = 1+ g phi. The strongest bound on the coupling parameter g is derived from the weak equivalence principle tests, which impose g < 1.6 x 10(-17) GeV-1. This constraint is strong enough to rule out this class of models as a cause for an observable cosmological variation of the fine structure constant unless a chameleon mechanism is implemented. Also, we argue that a similar coupling occurs in chameleon cosmology, another candidate dark mater particle, and we estimate the cosmological consequences by both effects. It should be clarified that this class of models is not necessarily ruled out in the presence of a chameleon mechanism which can freeze the dynamics of the scalar field in high density laboratory regions.