  Many modifications of gravity introduce new scalar degrees of freedom, and in
such theories matter fields typically couple to an effective metric that
depends on both the true metric of spacetime and on the scalar field and its
derivatives. Scalar field contributions to the effective metric can be
classified as conformal and disformal. Disformal terms introduce gradient
couplings between scalar fields and the energy momentum tensor of other matter
fields, and cannot be constrained by fifth force experiments because the
effects of these terms are trivial around static non-relativistic sources. The
use of high-precision, low-energy photon experiments to search for conformally
coupled scalar fields, called axion-like particles, is well known. In this
article we show that these experiments are also constraining for disformal
scalar field theories, and are particularly important because of the difficulty
of constraining these couplings with other laboratory experiments.
