The LHCb (Large Hadron Collider beauty) collaboration at CERN’s Large Hadron Collider in Switzerland has reported the observation of a doubly charmed particle. The particle, called the Ξcc++, is a baryon containing two charm quarks and one up quark.
The doubly charmed baryon Ξcc++ contains two charm quarks and one up quark. Image credit: Daniel Dominguez, CERN.
Nearly all the matter that we see around us is made of baryons, which are common particles composed of three quarks, the best-known being protons and neutrons.
But there are six types of existing quarks, known as flavors (up, down, strange, charm, top, and bottom), and theoretically many different potential combinations could form other kinds of baryons.
The existence of doubly charmed baryons was already known to be a possibility in the 1970s, after the discovery of the charm quark.
In the early 2000s, the observation of a similar particle, Ξcc+ (3520), was reported by Fermilab’s SELEX collaboration.
However, the discovery was not confirmed by subsequent experiments and the measured properties of the particle are not compatible with those of the newly-discovered Ξcc++ baryon.
Ξcc++ was identified via its decay into a Λc+ baryon and three lighter mesons K–, π+ and π+.
Its mass is about 3621 MeV, which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed quark content.
“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,” said Dr. Giovanni Passaleva, new spokesperson of the LHCb collaboration.
“Such particles will thus help us improve the predictive power of our theories.”
“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,” added Dr. Guy Wilkinson, former spokesperson of the collaboration.
Measuring the properties of Ξcc++ will help to establish how a system of two heavy quarks and a light quark behaves.
Important insights can be obtained by precisely measuring production and decay mechanisms, and the lifetime of this new particle.
The observation of the Ξcc++ in LHCb raises the expectations to detect other representatives of the family of doubly-heavy baryons.
“This discovery opens a new field of particle physics research,” the LHCb physicists said.
“An entire family of doubly charmed baryons related to the Ξcc++ is predicted, and will be searched for with added enthusiasm.”
“Furthermore, other hadrons containing different configurations of two heavy quarks, for example two beauty quarks or a beauty and charm quark, are waiting to be discovered.”
“Measurements of the properties of all these particles will allow for precise tests of QCD, the theory of strong interactions, in a unique environment.”
The LHCb collaboration has submitted a paper reporting these findings to the journal Physical Review Letters.
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R. Aaij et al (LHCb collaboration). 2017. Observation of the doubly charmed baryon Ξcc++. Phys. Rev. Lett, submitted for publication; CERN-EP-2017-156 / LHCb-PAPER-2017-018
