Searching for Light Dark Matter with the SLAC Millicharge Experiment

Understand

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.