Excited quarks

Excited quarks are a hypothetical state in which (a) quarks contain within them smaller particles and (b) energy is added to the quarks’ hypothetical constituents and is thus added to the quark. Since E = mc2, more energy in the quarks means they have a higher mass. Therefore scientists look for high-mass quarks that decay by emitting a photon and a regular quark.

It is well known that the Standard Model of particle physics is incomplete and that it is an approximation of a deeper and more fundamental theory. One idea that might lead the way to a better understanding of the universe is that the quarks and leptons, particles now treated as point-like, are actually composite and made of even smaller particles. Given that the quarks and leptons have electrical charge, it follows that at least some of these hypothetical components also have electrical charge. And where electrical charge exists, the photon must follow. This is because photons are emitted by electrically charged particles.

If quarks and leptons contain smaller objects, these objects are bound together with some sort of force. Further, since we know that the quarks and leptons act very much like point-like particles, this force must be very strong. Still, it is possible that if you hit a quark or lepton hard enough, you might be able to add energy to the constituents. Physicists call these energy-added particles “excited” quarks and leptons. The hypothetical constituents would somehow radiate that added energy and return to the quiescent states that are the familiar quarks and leptons. This is kind of like hitting a hornet’s nest, which will cause a swarm of hornets to fly around before the insects return to a quiet state.

If the constituent particles radiate energy, the quark might emit photons. The resulting signature of this excited quark would be a regular quark emitted with a lot of energy accompanied by the emitted photon. Since quarks turn into jets, the experimental signature is a jet and a photon. CMS scientists looked for this signature to try to find excited quarks. While no evidence was found, researchers were able to set limits on the energy scale at which excited quarks can be made. With the resumption of operations of the LHC in the spring of 2015, scientists will be able to look for even more massive excited quarks.

Don Lincoln

Sushil Chauhan of the University of California, Davis played an important role in this analysis.
These US CMS members are part of the FPIX Mechanics Group, responsible for the design and construction of the support and cooling structure of the Phase 1 upgrade of the CMS forward pixel detector.