Observation of doubly-charmed baryons at the LHCb experiment
Journal: Physical Review Letters
Publication Date: 11 September, 2017
Department of: Physics and Astronomy
Discovery of the first doubly-charmed baryon Ξcc++
Researchers at the University of Manchester working in the LHCb collaboration at the Large Hadron Collider, CERN have played a role in the first observation of the Ξcc++ (Xicc++) particle – a new baryon containing two charm quarks and one up quark. The existence of baryons with two heavy quarks was predicted for many years, but this is the first unambiguous experimental confirmation. The mass of the newly identified particle is about 3621 MeV, which is almost four times heavier than the proton, because it contains two charm quarks.
The observation of this new baryon proved to be challenging and has been made possible owing to the high production rate of heavy quarks at the LHC and to the unique capabilities of the LHCb experiment, which can identify the decay products with excellent efficiency. The Ξcc++ baryon was identified via its decay into a Λc+baryon and three lighter mesons K–, π+ and π+. This result is based on latest data recorded at the LHC in 2015-16, and confirmed with earlier data.
This discovery is only the first step towards understanding the behaviour of a system with two heavy and one light quark. The mass of the Ξcc++ was measured, in agreement with the Standard Model theoretical expectations. As next steps, the life time of this particle will be determined, and other decay modes will be searched for.
- The baryons like Λ, Ξ contain three valence quarks, the meson particles like K, π contain two valence quarks.
- The quarks are fundamental constituents of matter that can be studied as part of a composite particle due to a phenomenon called colour confinement.
- Previously only baryons with zero or one heavy quarks were known, but particles with up to three heavy quarks are predicted in the decades-old quark model.
- The composite particles can contain up to five valence quarks that determine the quantum numbers of the particle, and infinite amount of virtual (aka sea) quarks.
- The precision studies of the doubly-heavy charm baryons, and other composite particles (tetraquarks, pentaquarks) containing up to five valence quarks is possible because of the excellent capabilities of the heavy-flavour experiment LHCb.