Double The Charm For CERN’s LHCb Experiment That Discovers A New Type Of Baryon

The Large Hadron Collider (LHC) has done it again. This time, the largest particle accelerator in the world — a 17-mile tunnel under the French-Swiss border — has found a new type of baryon that contains two heavy quarks of the charm variety, the first ever detection of a baryon which has more than one heavy quark.

Baryons are subatomic particles, and along with mesons, make up the hadron family of particles. While mesons are made up of a quark and an antiquark, baryons (such as protons and neutrons) are composed of three quarks, of which at least two are light. There are currently six known types, or flavors, of quarks — three light and three heavy. Almost all matter that we experience or encounter is baryonic in nature.

Called Xi-cc++, the newly detected particle contains two charm quarks — one of the three known heavy flavors — and one up quark — a light flavor. Its existence had long been postulated by theories, but an unambiguous detection had eluded scientists for many years. It was finally observed by LHCb, one of the general experiments at the LHC.

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In a statement Thursday by the European Organization for Nuclear Research (CERN) that announced the detection, Giovanni Passaleva, spokesman for LHCb, said: “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.”

Xi-cc++ has a mass of 3,621 MeV, about four times heavier than a proton. Other properties, such as its lifetime or production and decay mechanisms, have not yet been established, but the system of two heavy quarks and one light quark is expected to behave differently from other baryons.

“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, former spokesman of the LHCb collaboration, said in the statement.

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LHC produces a large number of heavy quarks, making it possible for LHCb to detect the new particle by identifying its decay products. Xi-cc++ was identified by the particles it decayed into — another baryon and three mesons. The results were based on the 13 TeV of data that was recorded during run 2 of the LHC, and confirmed by 8 TeV data from run 1.

The observation of Xi-cc++ has raised expectations for the detection of more double-heavy quark baryons at LHC, and researchers there are going to start looking for them.

A paper titled “Observation of the doubly charmed baryon Xi-cc++” that described the particle and its detection is submitted for publication in the journal Physical Review Letters.