The mQ Experiment

The Millicharged Particle Search (mQ) was an experiment to search for unconventional, millicharged elementary particles.

A millicharged elementary particle would have an electric charge three or more factors of ten smaller than the charge of the electron. We do not have a deep understanding of charge quantization; as far as we now know, theory does not exclude the existence of millicharged particles. Experimental bounds, which come from precise QED checks and high-energy beam-dump experiments, exclude q/e > 10**-2 over a wide range of masses. Very small charges (q/e < 10**-6) are disfavored by astrophysical arguments, but particles with 10**-6 < q/e < 10**-3 could exist in nature. Theorists have suggested that millicharged elementary particles may consitute the so-called dark matter or account for other experimental anomalies.

This experiment ran parasitically during routine SLC operation, which requires production of positron beams. Electroproduction is the mechanism for producing millicharged particles, and is fully calculable, allowing a definitive search over a wide range of charge and mass. Unlike other beam-dump experiments, this search exploited a low-energy signature in the ionization and/or excitation of atoms by using a scintilation detector to search for very small energy depositions. Understanding the scintillation mechanism at the single-photon level was an important component of the experiment(1).

The collaborators were:

• Baggs, Ron
• Ballam, Joseph
• Ecklund, Stan
• Fertig, Chad
• Jaros, John
• Kase, Kenneth
• Kulikov, Artem
• Langeveld, Willy
• Leonard, Robert
• Marvin, Tom
• Nakashima, Tom
• Nelson, W. Ralph
• Odian, Allen
• Pertsova, Maria
• Prinz, Alyssa
• Putallaz, Gerard
• Weinstein, Amanda

• Field, Clive
• Loomer, Jerry
• Swartz, Morris

For further information about the mQ experiment , contact John Jaros.

The page was compiled by Joan Winters with help on image scanning from Patrick Clancey, and modified by Willy Langeveld