LHCb experiment discovers a new pentaquark

Mar 26, 2019

The LHCb collaboration has announced the discovery of a new pentaquark particle. The particle, named Pc(4312)+, decays to a proton and a J/ψ particle (composed of a charm quark and an anticharm quark). This latest observation has a statistical significance of 7.3 sigma, passing the threshold of 5 sigma traditionally required to claim a discovery of a new particle.

In the conventional quark model, composite particles can be either mesons formed of quark–antiquark pairs or baryons formed of three quarks. Particles not classified within this scheme are known as exotic hadrons. When Murray Gell-Mann and George Zweig proposed the quark model in their 1964 papers, they mentioned the possibility of exotic hadrons such as pentaquarks, but it took 50 years to demonstrate their existence experimentally. In July 2015, the LHCb collaboration reported the Pc(4450)+ and Pc(4380)+ pentaquark structures. The new particle is a lighter companion to these pentaquark structures and its existence sheds new light into the nature of the entire family.

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered at LHCb. The five quarks might be assembled into a meson (one quark and one antiquark) and a baryon (three quarks), weakly bound together (Image: Daniel Dominguez/CERN)

The analysis presented today at the Rencontres de Moriond quantum chromodynamics (QCD) conference used nine times more data from the Large Hadron Collider than the 2015 analysis. The data set was first analysed in the same way as before and the parameters of the previously reported Pc(4450)+ and Pc(4380)+ structures were consistent with the original results. As well as revealing the new Pc(4312)+ particle, the analysis also uncovered a more complex structure of Pc(4450)+ consisting of two narrow overlapping peaks, Pc(4440)+ and Pc(4457)+, with the two-peak structure having a statistical significance of 5.4 sigma. More experimental and theoretical study is still needed to fully understand the internal structure of the observed states.

Read more on the LHCb website.

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