Searching for Light Dark Matter with the SLAC Millicharge Experiment
  New sub-GeV gauge forces ("dark photons") that kinetically mix with the
photon provide a promising scenario for MeV-GeV dark matter, and are the
subject of a program of searches at fixed-target and collider facilities around
the world. In such models, dark photons produced in collisions may decay
invisibly into dark matter states, thereby evading current searches. We
re-examine results of the SLAC mQ electron beam dump experiment designed to
search for millicharged particles, and find that it was strongly sensitive to
any secondary beam of dark matter produced by electron-nucleus collisions in
the target. The constraints are competitive for dark photon masses in the ~1-30
MeV range, covering part of the parameter space that can reconcile the apparent
(g-2)_{\mu} anomaly. Simple adjustments to the original SLAC search for
millicharges may extend sensitivity to cover a sizable portion of the remaining
(g-2)_{\mu} anomaly-motivated region. The mQ sensitivity is therefore
complementary to on-going searches for visible decays of dark photons. Compared
to existing direct detection searches, mQ sensitivity to electron-dark matter
scattering cross sections is more than an order of magnitude better for a
significant range of masses and couplings in simple models.
