Meet the pentaquark, just spotted at CERN’s Large Hadron Collider

For decades, physicists have looked for the pentaquark -- a type of subatomic particle long theorized to exist but never seen, despite numerous false alarms.

This week, scientists at the European Organization for Nuclear Research, or CERN, announced that they had finally found their elusive quarry.

In a paper submitted to the journal Physical Review Letters, the organization’s LHCb collaboration described how it detected a signal in data from proton-proton collisions at the Large Hadron Collider near Geneva and eliminated various other explanations through a series of tests.

“We have examined all possibilities for these signals, and conclude that they can only be explained by pentaquark states,” Syracuse University physicist and collaboration member Tomasz Skwarnicki said in a statement.

In grade school, students learn that atoms are made up of protons, neutrons and electrons. Protons and neutrons, in turn, are each made up of three even smaller particles known as quarks.

Scientists have known since the 1960s that three-quark particles (known as baryons) and two-quark particles (known as mesons) existed. But for the last 50 years or so, said UC San Diego physicist Vivek Sharma, many believed that it must be theoretically possible to have other combinations too.

“You get four quarks together, it’s a tetraquark. You get five quarks, it’s a pentaquark,” said Sharma, who is part of the Large Hadron Collider’s CMS experiment, one of the two groups that observed the Higgs boson in 2012. (He was not involved in the pentaquark search.)

The LHCb team found the pentaquark while observing the decay of a baryon known as Lambda B. They didn’t observe the unstable pentaquark directly but rather measured the products of its disintegration, working backward to figure out that it must have been present.

Sharma said it was an “exciting and very convincing experimental breakthrough” that will help physicists better understand how quarks interact.

“There are theories--when you have five quarks held together, do they stay in this tiny confined region tightly bound to each other via strong interaction” -- the force that binds the quarks in protons and neutrons -- “or are they a loosely bound system of a baryon and a meson interacting relatively weakly? It’s understanding the detailed dynamics of how quarks talk to each other,” he said.

Sharma added that is was likely that pentaquarks exist in superdense celestial objects like neutron stars, produced after the gravitational collapse of massive stars, and that producing and studying pentaquarks on Earth could help researchers understand the physics of neutron stars.

Robert Cousins, a particle physicist at UCLA and another CMS collaborator, said that the discovery highlighted the wide range of questions the Large Hadron Collider can be used to address. The LHCb group studied fundamental forces in very light particles, he noted, while Higgs hunters work with far heavier ones.

“This reminds us of the breadth of the research that’s done with the LHC,” he said.

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