Frontiers Physics World  August 2017

LHCb sees doubly charmed baryon

Twice as charming LHCb’s new baryon. (CERN)

A baryon containing two charm quarks has been detected for the first time. The discovery was made by physicists working on the LHCb experiment at the Large Hadron Collider (LHC) at CERN.

Weighing in at 3621 MeV, the Ξ+c+c particle – which also contains an up quark – has about the same mass as a helium-3 nucleus. Although it is predicted by the Standard Model of particle physics, the Ξ+c+c discovery and subsequent study should give important information about how to calculate the properties of particles made of quarks.

Baryons – particles containing three quarks – include the familiar proton and neutron, which both comprise up and down quarks. There are also more exotic but less stable baryons containing charm, strange and bottom quarks. However, it is extremely difficult to calculate the properties of baryons using quantum chromodynamics – the theory that describes the strong force through which quarks interact.

Fortunately, the internal structure of Ξ+c+c can be imagined as a planet (the relatively low-mass up quark) orbiting a binary star system (the two much heavier charm quarks). According to Giovanni Passaleva at LHCb, this configuration makes it relatively straightforward to use the various QCD calculation schemes to work out the mass and other properties of Ξ+c+c. Comparing these calculations to measurements made at the LHCb should allow physicists to improve how QCD calculations are done.

“Finding a doubly heavy-quark baryon is of great interest as it will provide a unique tool to further probe QCD,” says Passaleva. “Such particles will thus help us improve the predictive power of our theories.”

The Ξ+c+c was created by proton collisions in both the 7 TeV and 13 TeV runs of the LHC. It was identified in the LHCb via its decay into a Λ+c baryon and three lighter mesons: the K, π+ and π. The statistical significance of the measurement is far in excess of 5σ, which is the “gold standard” for a discovery in particle physics.

Passaleva says that the LHCb team now wants to take a closer look at how the Ξ+c+c is produced in the LHC, as well as examine the particle’s lifetime and decay mechanisms. He points out that LHCb is eminently suited for this task because it is designed to make very precise measurements of the particle decays of interest.