  Some of the simplest modifications to general relativity involve the coupling
of additional scalar fields to the scalar curvature. By making a Weyl rescaling
of the metric, these theories can be mapped to Einstein gravity with the
additional scalar fields instead being coupled universally to matter. The
resulting couplings to matter give rise to scalar fifth forces, which can evade
the stringent constraints from local tests of gravity by means of so-called
screening mechanisms. In this talk, we derive evolution equations for the
matrix elements of the reduced density operator of a toy matter sector by means
of the Feynman-Vernon influence functional. In particular, we employ a novel
approach akin to the LSZ reduction more familiar to scattering-matrix theory.
The resulting equations allow the analysis, for instance, of decoherence
induced in atom-interferometry experiments by these classes of modified
theories of gravity.
