Subatomic signatures

Charged particles passing through a detector leave behind tracks laden with information in their lengths, trajectories, curvatures and thicknesses. Image source

When the idea of quarks was first introduced in the 1960s, one of its stranger aspects was that quark charges must be one-third and two-thirds that of an electron. Up to that point, all known particles had −2, −1, 0, +1 or +2 times as much electric charge as the electron, a pattern that suggests that charge is quantized (only integer multiples are allowed). Quarks, which never appear alone, tell us that the basic unit is three times smaller. But solitary particles with such small charges have never been observed.

A solitary quark, if it could float through space on its own, would be a fractionally charged particle. The reason we have never seen lone quarks is now known: Quarks are bound together by a force so strong that any attempt to separate them simply creates more quarks. However, there is no obvious link between the fractional charge of the quarks and the strong force holding them together. In principle, there could be other particles with fractional charges and no strong force.

Physicists have been searching for free-floating, fractionally charged particles for decades. These particles, if they exist, might have eluded discovery by being too massive to be created in previous accelerators. Thus CMS scientists analyzed LHC data to see if its much higher energy is capable of producing heavy particles with fractional charge.

This analysis uses a very different technique from most studies at particle colliders. Usually, physicists measure the lifetime and energy of particles from the lengths, trajectories and curvatures of the tracks they leave behind as they swoop through the detector. To determine the strength of a particle’s charge, however, one must use the thickness as well, the boldness of the signature’s stroke.

In a bubble chamber like the ones used to look for free quarks in the 1960s, a fractionally charged particle would have produced fewer bubbles along its path in the fluid, drawing a weaker line. In a modern experiment like CMS, a fractionally charged particle would produce smaller electric signals as it passes through strips of silicon. CMS scientists had to find voltage blips nine times weaker than those from other particles.

After a thorough search, the analyzers have concluded that, if free-floating, fractionally charged particles exist, their masses must be greater than a quarter of a TeV—heavier than the heaviest known particle—to avoid being created by the LHC.

—Jim Pivarski

The U.S. physicists pictured above contributed greatly to the search for fractionally charged particles.
Future detectors will identify charged particle tracks by their signals in synthetic diamond, rather than silicon. The Rutgers University physicists pictured above developed the CMS Beam Conditions Monitor, currently in use, and the CMS Pixel Luminosity Telescope, which will be installed during the upcoming shutdown. The latter will be the first diamond-based tracking device used in particle physics.