Searching for light dark matter with the SLAC millicharge experiment

Area of Science:

• Particle Physics
• Cosmology
• Astrophysics

Background:

• Sub-GeV gauge forces, or "dark photons," kinetically mixing with photons are a leading candidate for MeV-GeV dark matter.
• Dark photons may decay invisibly into dark matter, evading current detection methods.
• The anomalous magnetic dipole moment of the muon (g-2)(μ) suggests new physics, potentially linked to dark photons.

Purpose of the Study:

• To reexamine the SLAC mQ electron beam dump experiment for sensitivity to dark matter produced by dark photons.
• To constrain dark photon models, particularly in the ~1-30 MeV mass range relevant to the (g-2)(μ) anomaly.
• To compare the sensitivity of the mQ experiment to existing direct-detection dark matter searches.

Main Methods:

• Analysis of data from the SLAC mQ experiment, originally designed to search for millicharged particles.
• Modeling of dark matter production via electron-nucleus collisions and subsequent dark photon interactions.
• Comparison of experimental constraints with theoretical predictions for dark photon properties and dark matter interactions.

Main Results:

• The SLAC mQ experiment provides strong constraints on dark matter produced by dark photons in the ~1-30 MeV mass range.
• These constraints cover a portion of the parameter space that could explain the (g-2)(μ) anomaly.
• The mQ experiment's sensitivity to electron-dark matter scattering cross sections is superior to direct-detection searches for certain mass and coupling ranges.

Conclusions:

• The SLAC mQ experiment offers a complementary approach to searching for dark photons, particularly those decaying invisibly.
• Simple modifications to the mQ search could enhance sensitivity to the (g-2)(μ) anomaly-motivated region.
• The mQ experiment's results significantly improve upon existing limits for dark matter interactions in specific parameter spaces.