Search for Millicharged Particles at SLAC

TL;DR

This paper reports on a SLAC experiment that searches for millicharged particles with very small electric charges and masses between 1 and 100 MeV/c^2, setting new upper limits on their possible charge.

Contribution

It introduces a specialized experiment at SLAC that effectively detects and constrains the existence of millicharged particles in a previously unexplored parameter space.

Findings

Established new upper limits on millicharged particle charge at 95% confidence.

Excluded certain mass and charge regions for millicharged particles.

Demonstrated the experiment's sensitivity to rare ionization events.

Abstract

Particles with electric charge q < 10^(-3)e and masses in the range 1--100 MeV/c^2 are not excluded by present experiments. An experiment uniquely suited to the production and detection of such "millicharged" particles has been carried out at SLAC. This experiment is sensitive to the infrequent excitation and ionization of matter expected from the passage of such a particle. Analysis of the data rules out a region of mass and charge, establishing, for example, a 95%-confidence upper limit on electric charge of 4.1X10^(-5)e for millicharged particles of mass 1 MeV/c^2 and 5.8X10^(-4)e for mass 100 MeV/c^2.