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 reexamine 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 similar to 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 ongoing searches for visible decays of dark photons. Compared to existing direct-detection searches, mQ sensitivity to electron-darkmatter scattering cross sections is more than an order of magnitude better for a significant range of masses and couplings in simple models.