Pentaquark seen at CERN

A schematic showing loosely bound and tightly bound pentaquark systems.

ON July 14, the LHCb experiment at the European Organisation for Nuclear Research’s (CERN) Large Hadron Collider reported the discovery of an exotic class of particles known as pentaquarks, composite objects made of five quarks. The findings are due to be published in the journal Physical Review Letters.

According to the quark model, proposed in 1964, quarks are the basic building blocks of matter, and all the particles are made of either two or three quarks. For example, particles called mesons are made of a quark and an antiquark, while other particles called baryons, such as the familiar protons and neutrons, contain three quarks. The quark model allows for particles composed of four or five quarks, but these had not been seen. In 2013, some experimental groups claimed to have discovered tetraquarks, composite particles with four quarks.

“The pentaquark is not just any new particle,” said LHCb spokesperson Guy Wilkinson. “It represents a way to aggregate quarks… in a pattern that has never been observed before in over 50 years of experimental searches. Studying its properties may allow us to understand better how ordinary matter, the protons and neutrons from which we’re all made, is constituted.”

LHCb researchers looked for pentaquark states by examining the decay of a baryon known as “Lambda b” into three other particles, a J-psi particle, a proton and a charged kaon. Studying the spectrum of masses of the J-psi and the proton revealed that intermediate states were sometimes involved in their production. According to the CERN release, which quotes LHCb physicist Tomasz Skwarnicki of Syracuse University, these states can only be explained by pentaquark objects.

Earlier experiments that searched for pentaquarks were inconclusive. Where the LHCb experiment differs is that it has been able to look for pentaquarks from many perspectives, with all pointing to the same conclusion. The next step in the analysis will be to study how the quarks are bound together within the pentaquarks.

“The quarks could be tightly bound,” said LHCb physicist Liming Zhang of Tsinghua University, “or... loosely bound in a sort of meson-baryon molecule, in which the meson and baryon feel a residual strong force similar to the one binding protons and neutrons to form nuclei.”