Constraints on scalar coupling to electromagnetism

TL;DR

This paper reviews constraints on a scalar field's non-minimal coupling to electromagnetism, focusing on experimental bounds, especially from equivalence principle tests, and discusses implications for cosmology and dark matter models.

Contribution

It provides the strongest experimental bounds on the coupling parameter and analyzes the viability of such models with chameleon mechanisms in cosmology.

Findings

Weak equivalence principle tests constrain the coupling to g < 1.6 x 10^{-17} GeV^{-1}

The coupling is unlikely to cause observable variations in the fine-structure constant without a chameleon mechanism

Chameleon mechanisms can suppress scalar field effects in high-density environments.

Abstract

We review a possible non-minimal coupling (dilatonic) 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 $B_F (\phi) = 1 + g\phi$. The strongest bound on the coupling parameter $g$ is derived from weak equivalence principle tests, which impose $g < 1.6 \times 10^{-17}GeV^{-1}$. 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.