CERN Physicists Discover New Pentaquark Particle

Physicists from CERN’s Large Hadron Collider beauty (LHCb) Collaboration announced this week the discovery of a new pentaquark particle, Pc(4312)+, decaying to a proton and a J/ψ meson, a particle composed of a charm quark and a charm antiquark.

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered by the LHCb Collaboration; the five quarks might be tightly bonded or assembled differently. Image credit: CERN.

Illustration of the possible layout of the quarks in a pentaquark particle such as those discovered by the LHCb Collaboration; the five quarks might be tightly bonded or assembled differently. Image credit: CERN.

In the conventional quark model, strongly interacting particles known as the hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons.

In his fundamental 1964 paper, in which he proposed the quark model, the American physicist Murray Gell-Mann mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration.

It took 50 years, however, for scientists to obtain unambiguous experimental evidence of the existence of these exotic hadrons.

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.

“Until now, we had thought that a pentaquark was made up of five elementary particles (quarks), stuck together. Our findings prove otherwise,” said Syracuse University’s Professor Tomasz Skwarnicki, member of the LHCb Collaboration.

The new analysis used almost 10 times more data from the Large Hadron Collider than the 2015 analysis.

“LHCb’s latest data utilized an energy beam that was nearly twice as strong. This method, combined with more refined data-selection criteria, produced a greater range of proton collisions,” Professor Skwarnicki said.

“It also gave us 10 times more data and enabled us to observe pentaquark structures more clearly than before. What we thought was just one pentaquark turned out to be two narrow ones, with little space between them.”

The new data set was first analyzed 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)+.

More experimental and theoretical study is still needed to fully understand the internal structure of the observed states.

“Pentaquarks may not play a significant role in the matter we are made of, but their existence may significantly affect our models of the matter found in other parts of the Universe, such as neutron stars,” Professor Skwarnicki said.

The LHCb physicists presented their results this week at the QCD High Energy Interactions session of the 54th Rencontres de Moriond Conference in La Thuile, Italy.

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Tomasz Skwarnicki et al (LHCb Collaboration). Hadron spectroscopy and exotic states at LHCb. Morion QCD 2019

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