Researchers at the Large Hadron Collider announced the discovery of a particle that had been predicted in theory, but never seen experimentally.
Dubbed Xi cc, the newly-identified particle is a kind of baryon, a subatomic particle comprised of three quarks. Virtually all the matter we can see is made of baryons, with protons and neutrons being the most widely-known baryons.
Quarks are tinier particles that come in six “flavors”, two standard types that are light and four heavier varieties. For years, Physicists have been on the lookout for baryons with two heavy quarks and they finally found one in Xi cc, which has two (heavy) charm quarks and one up (light) quark.
Found to be exactly as predicted, the mass of the newly discovered particle is approximately 3621 MeV, which is nearly four times heavier than a proton. If the mass of Xi cc was found to be much lower, it would have revealed a major problem in the Standard Model. However, that didn't occur; the particle was as expected.
The unique particle wasn't observed directly by the LHCb experiment, mainly because it lasts for just a small fraction of a second. In this situation, Scientists recognized the Xi cc particle by the products it broke down into: a particle known as the lambda baryon, a kaon and two pions.
Finding a doubly heavy-quark baryon is of great interest as it will provide a unique tool to further probe quantum chromodynamics, the theory that describes the strong interaction, one of the four fundamental forces. Such particles will thus help us improve the predictive power of our theories.
Giovanni Passaleva, Spokesperson, LHCb collaboration
The new particle was particularly unique for the way in which its quarks appeared to behave. Quarks don't have positions, but instead act more like waves. However, that wasn’t what the LHCb team found.
In contrast to other baryons, in which the three quarks perform an elaborate dance around each other, a doubly heavy baryon is expected to act like a planetary system, where the two heavy quarks play the role of heavy stars orbiting one around the other, with the lighter quark orbiting around this binary system.
Guy Wilkinson, a Former LHCb Spokesperson
Through studying Xi cc, Physicists hope to determine how a system of two heavy quarks and one light quark acts. Critical insights can be acquired by accurately calculating the formation of the particle and breakdown mechanisms, as well as its lifetime. This research could lead to significant revelations about the “strong force” that holds atoms together.
The LHCb team said the discovery with this new baryon turned out to be difficult and was made possible by the high generation rate of heavy quarks at the LHC. The organization said its distinctive abilities to identify decay products with superb efficiency also helped to make the discovery possible. They added that they now hope to find more doubly-heavy baryons.
While the discovery of Xi cc appeared to confirm the Standard Model, a previous LHCb experiment did show potential deviations from the foundational particle physics theory.
In that study, LHCb Researchers examined the decays of B0 mesons that produce an excited kaon and either two electrons or two muons. While a muon is 200 times heavier than an electron, the Standard Model says its interactions are essentially identical to those of the electron, a quality referred to as lepton universality.
Lepton universality states that electrons and muons should be generated with the same probability in this particular B0 decay, with a small variation due to mass difference. However, LHCb Researchers found decays involving muons happen less often than those involving electrons.
While potentially groundbreaking, the disparity with the Standard Model occurs at a deviation not adequate to make a firm conclusion, the LHC Researchers said in a statement.