Novel Screening with Two Bodies: Summing the ladder in disformal
  scalar-tensor theories
  When augmenting our cosmological models or gravitational theories with an
additional light scalar field, any coupling between matter and this scalar can
affect the orbital motion of binary systems. Ordinarily, the new force mediated
by the scalar can be naturally the same order of magnitude as the usual
gravitational force and therefore is tightly constrained. We show that a
disformal coupling between the scalar and matter can lead to a novel screening
mechanism in which these fifth forces are suppressed by several orders of
magnitude at sufficiently small separations and large relative velocities (such
as solar system scales). This is a result of resumming a class of ladder
diagrams, which suppresses the propagation of scalar signals between the two
bodies. Moreover, we are able to relate potential ambiguities in this
resummation to non-perturbative effects (which are invisible to perturbation
theory). As a result, solar system tests and future gravitational wave
observations can now be used to place meaningful constraints on scalar-tensor
theories with disformal couplings. We exemplify this using observational bounds
on the precession of planetary orbits.
