Searching for Light Dark Matter with the SLAC Millicharge Experiment

TL;DR

This paper reinterprets the SLAC mQ experiment data to set new constraints on light dark matter and dark photons, showing it is sensitive to parameter space relevant to the muon g-2 anomaly and surpasses some direct detection methods.

Contribution

It demonstrates that the SLAC mQ experiment can be used to probe dark matter models with dark photons, extending the experimental sensitivity to new regions of parameter space.

Findings

SLAC mQ experiment constrains dark photon masses 1-30 MeV.

Sensitivity covers part of the muon g-2 anomaly region.

Improved analysis could enhance sensitivity to dark matter signals.

Abstract

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.