Pentaquark Discovered at LHC by Accident


It’s a particle so elusive that even the world’s largest physics experiment could only discover it by accident. The pentaquark has at last been found. While finding the particle hinged on a stroke of good fortune, the discovery marks another milestone for CERN’s Large Hadron Collider (LHC), the world’s largest and most powerful particle collider.

“It’s better to be lucky than good, but it’s really good to be both good and lucky,” said Sheldon Stone, a physicist at Syracuse University in New York and co-author of the recent study, which was submitted to Physical Review Letters.

“It’s not something we’ve been actively looking for,” says Patrick Koppenburg, a Physics Coordinator at the Large Hadron Collider at CERN near Geneva, Switzerland. “It’s more something we’ve stumbled across. Pentaquarks have been reported several times in the past,” says Koppenburg, “but they all appeared to be fakes.”

The pentaquark represents a fundamentally new arrangement of quarks, indivisible particles that are some of the building blocks of matter. The discovery fits into scientists’ ever-better understanding of the Universe’s menagerie of particles. With names such as kaons, psions and charmonium, they may sound as if they power comic book villains’ doomsday devices. But physicists have figured out that many of these are composed of building blocks called quarks. Quarks come in six different “flavors,” or types: up and down; top and bottom; and charm and strange.

The pentaquark was observed by studying the decay of particles called Lambda b baryons, which normally occurs in one smooth step. However, the physics of the strong force, which governs the way quarks behave, means there are sometimes intermediate states along the way. During one such not so smooth decay, an elusive five-quark particle popped up with a signature that tallied with that of the theorised pentaquark.

“This is a decay that physicists know and love,” says Guy Wilkinson, the spokesperson for the LHCb collaboration, “so in that sense it was certainly a serendipitous discovery

When these quarks buddy up, usually in groups of two or three, they give rise to the bigger subatomic particles we see in nature. Protons, for example, are each made of two up quarks and one down quark. But why stop there? When the physicist Murray Gell-Mann first theorized quarks’ existence in 1964, he suggested that arbitrarily large numbers of them could coalesce to form bigger particles. Theoretically, there wasn’t a reason why there couldn’t be four-quark “tetraquarks” – or five-quark “pentaquarks,” for that matter. So ever since, scientists have been on the hunt, hoping to temporarily prod tetraquarks and pentaquarks into existence with increasingly powerful particle colliders like the LHC. Because the particle decays almost immediately, more experiments will be needed to determine further information about its size and mass.

This article originally appeared on, written by Feiryred.

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