Looking in many boxes

Searching for particles smaller than any previously discovered requires looking into lots of different boxes. An earlier search using leptons was unsuccessful, so this analysis turned to quarks.

In February, I wrote an article that described a search for the smallest building blocks of matter. Currently, the best description we have of the subatomic realm is the Standard Model, and this theory treats particles called quarks and leptons as having no size at all. On the other hand, the history of physics is full of particles that were once thought to be the smallest of the small and turned out not to be: Atoms, atomic nuclei and protons are the best known ones.

If there are particles smaller than the ones we know, then there is some sort of force that binds the smaller particles into the familiar quarks and leptons. When we begin to probe the smaller scale, we expect to encounter new physics, and the character of the data will change. Because these changes will be caused by contacting the new physical phenomena, these kinds of pursuits are called “contact interactions,” and the math treats the interactions as if the particles actually come into contact.

In the earlier article, I described how CMS scientists were searching for contact interactions in the production of leptons. This allows physicists to see if there is new physics to be found at the smallest scales when investigating electrons and muons.

There is no guarantee that leptons are the right place to search for new phenomena. Leptons may be sufficiently point-like that we can’t see contact interactions using our equipment. However, it is possible that quarks and gluons are less point-like than leptons. Accordingly, it is important also to check the data for contact interactions for quarks. CMS has looked for any evidence of new physics occurring at very small scales when quarks are scattered. No evidence for unexpected contact interactions was discovered.

—Don Lincoln

These US CMS scientists contributed to this analysis.
The 2013 annual US CMS meeting took place recently at Vanderbilt University. For any such event to run smoothly, postdocs, graduate students and undergraduates are enlisted to help with operations, including running projection systems, making copies and helping visitors solve all manner of unexpected problems. These Vanderbilt University physicists were instrumental in making the meeting a spectacular success.