The first detection of a baryon containing two charm quarks has been made by physicists working on the LHCb experiment at the Large Hadron Collider (LHC) at CERN. Weighing in at 3621 MeV, the Ξ++cc particle has about the same mass as a helium-3 nucleus. Although the particle – which also contains an up quark – is predicted by the Standard Model of particle physics, its discovery and subsequent study should give important information about how to calculate the properties of particles made up of quarks.

Baryons are particles with three quarks and include the familiar proton and neutron (comprising up and down quarks) as well as more exotic particles that can contain charm, strange and bottom quarks. Quarks interact via the strong force and this is described by the theory of quantum chromodynamics (QCD). However, the nature of the strong force makes it extremely difficult to calculate the properties of baryons using QCD.

Finding a doubly heavy-quark baryon is of great interest as it will provide a unique tool to further probe QCD
Giovanni Passaleva, LHCb

Binary star

The internal structure of Ξ++cc can be imagined as a planet (the relatively low-mass up quark) orbiting a binary star (the two much heavier charm quarks). According to LHCb's newly appointed spokesperson Giovanni Passaleva, this configuration makes it relatively straightforward to use the various QCD calculation schemes to work out the mass and other properties of Ξ++cc. 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++cc was created in 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.

Lifetime and decay

Passaleva says that the LHCb team now wants to take a closer look at how the Ξ++cc 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.

The discovery was announced today at the European Physical Society Conference on High Energy Physics in Venice and will be described in an upcoming paper in Physical Review Letters.