New Pentaquark Discovered by LHCb Collaboration Could Shed Light on the Nature of Pentaquarks

The discovery of a new pentaquark particle has been announced by the LHCb collaboration. Named P_c(4312)⁺, the particle decays into a proton and a J/ψ particle (formed of a charm quark and an anticharm quark).

The statistical significance of this most recent observation is 7.3 sigma, outperforming the threshold of 5 sigma conventionally needed to claim a new particle discovery.

According to the traditional quark model, composite particles can be mesons composed of quark–antiquark pairs or baryons composed of three quarks. Particles that cannot be categorized using this model are called exotic hadrons. Murray Gell-Mann, while proposing the quark model in his 1964 fundamental paper, pointed out the possibility of exotic hadrons like pentaquarks; however, it took 50 years to experimentally verify their existence.

The LHCb collaboration announced the existence of the P_c(4450)⁺ and P_c(4380)⁺ pentaquark structures in July 2015. The new particle is a lighter equivalent of these pentaquark structures. Its existence throws new light into the nature of the whole family.

In the analysis presented at the Rencontres de Moriond quantum chromodynamics (QCD) conference on March 26^th, 2019, nine times more data were used from the Large Hadron Collider compared to the 2015 analysis. First, the data set was analyzed the same way as earlier, and the parameters of the P_c(4450)⁺ and P_c(4380)⁺ structures reported earlier were consistent with the original results.

Apart from unraveling the new P_c(4312)⁺ particle, the analysis also revealed a highly complex structure of P_c(4450)⁺ including two narrow overlapping peaks—P_c(4440)⁺ and P_c(4457)⁺—where the two-peak structure had a statistical significance of 5.4 sigma. Additional theoretical and experimental study is still required to gain complete insights into the internal structure of the observed states.