Constraints on scalar coupling to electromagnetism
  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.
