Searching for particles with fractional charges

This plot shows exclusion limits at the 90 percent confidence level on the rate of downward-going fractionally charged particles versus inverse electric charge in units of 1/e, under the conservative assumptions. The blind (light gray solid) and improved, nonblind (black solid) analyses of the CDMS II experiment are compared with past cosmogenic searches MACRO (dashed lower-left), Kamiokande (X) and LSD (+).

You may have heard that particles have something called electric charge. Scientists often quantify this charge in unitless numbers. For example, protons have a charge of 1; electrons have a charge of -1. Particles are assigned charge numbers based on how much they have relative to an electron.

Some extensions to the Standard Model of particle physics predict the existence of fractionally charged particles. Such particles that come to us from the cosmos interact with the electrons in our Earth-bound particle detectors. The probability of that interaction depends on the amount of charge it has — the more it has, the more likely it is to interact with the detector’s electrons.

While the Cryogenic Dark Matter Search (CDMS-II) experiment was designed to focus on dark matter (as its name states), the experiment’s ability to detect smaller energy depositions gives it sensitivity to other types of new physics as well. CDMS is the first experiment to probe for particles arriving from outer space with fractional charges less than 1/6 of that of an electron.

CDMS-II operated germanium and silicon detectors in vertical stacks of six detectors. An energetic, fractionally charged particle could interact with all detectors in a single stack, creating a signature, a track, very different from that expected from dark matter particles or even from normal matter interactions.

This analysis relied on two main requirements. First, a fractionally charged particle candidate must have a reconstructed track that is consistent with a straight line. Second, each of the six observed energy depositions in a detector stack must be consistent with that expected for a particle with a given fractional charge. The expected background is reduced to almost zero by these requirements.

CDMS scientists observed no candidate events for fractionally charged particles, allowing us to set limits on the rate of downward-going fractionally charged particles for charges as small as 1/200th of an electron (see above figure).

The next generation SuperCDMS SNOLAB dark matter search experiment will also have greatly improved sensitivity to fractionally charged particles, thanks to lower energy thresholds and larger detectors. Perhaps, in a few years, we may know whether particles with charges less than 1/100th of the electron exist in our universe.

Joel Sander, University of South Dakota

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These are two of the many members of the CDMS-II collaboration who contributed to this search for fractionally charged particles. Left: Raymond Bunker from the South Dakota School of Mines and Technology. Right: Joel Sander from the University of South Dakota.