Discovery of three new Xic0 baryons at the LHCb experiment

University of Edinburgh colleagues who are working on the LHCb (Large Hadron Collider beauty) experiment at CERN (the European Organization for Nuclear Research) have discovered a system of three particles interpreted as three new excited Xic0 states. The Xic0 state is a baryon composed of a charm-, a strange- and a down-quark (csd).

The lightest of all baryons, the proton, which is the nucleus of the hydrogen atom, is composed of two up- and one down-quark (uud) while its neutral partner the neutron is composed of two down- and one up-quark (ddu). If one (or more) light quark is replaced by either a charm c or a beauty b heavy quark we obtain heavier charmed or beauty baryon particles. The three quarks can also be formed in their lowest-energy quantum mechanical state: the ground state. Like electrons in atoms, the quarks can be rearranged into excited states with different values of angular momentum and quark spin orientation.

LHCb physicists searched for excited Xic0 states in their decay into a Λc+ baryon and a K- meson. Three new excited states of the Xic0 baryon have been observed and are named Ξc(2923)0, Ξc(2939)0 and Ξc(2965)0. The numbers in brackets represent the measured masses of each state.

Emmy Gabriel, PhD student within the Particle Physics Experiment Research Group at Edinburgh who was leading the analysis, said:

It has been incredibly interesting to perform an analysis in the field of baryon spectroscopy, and I am excited to see how the community will respond to this discovery.

The observed system seems to be related to another system of baryons observed a few years ago which drew a lot of attention from the scientific community.

Dr Marco Pappagallo, LHCb research assistant within the Particle Physics Experiment Group reported:

This discovery probes the internal structure of the baryons and helps us to understand how quarks bind together inside the hadrons.

An upgrade of the LHCb experiment is ongoing with the contribution of the Edinburgh group into commissioning the Cherenkov detector, which is essential to identify the different types of particles involved in this discovery. The LHCb experiment plans to collect a much larger dataset in the upcoming years which will allow physicists to study the properties of these new states by measuring their spin and parities.

Prof Franz Muheim, leader of the LHCb team at the University of Edinburgh said:

With this avalanche of discoveries of new baryons with charmed and b-quarks, LHCb has established charmed and beauty baryon spectroscopy as an experimental topic.