Spin precession as a new window into disformal scalar fields
  We launch a first investigation into how a light scalar field coupled both
conformally and disformally to matter influences the evolution of spinning
point-like bodies. Working directly at the level of the equations of motion, we
derive novel spin-orbit and spin-spin effects accurate to leading order in a
nonrelativistic and weak-field expansion. Crucially, unlike the
spin-independent effects induced by the disformal coupling, which have been
shown to vanish in circular binaries due to rotational symmetry, the
spin-dependent effects we study here persist even in the limit of zero
eccentricity, and so provide a new and qualitatively distinct way of probing
these kinds of interactions. To illustrate their potential, we confront our
predictions with spin-precession measurements from the Gravity Probe B
experiment and find that the resulting constraint improves upon existing bounds
from perihelion precession by over 5 orders of magnitude. Our results therefore
establish spin effects as a promising window into the disformally coupled dark
sector.
