Los Angeles Times

Scientists report they’ve found the elusive pentaquark

- By Eryn Brown eryn.brown@latimes.com Twitter: @LATerynbro­wn

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

Last 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 knownas 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 tetra quark. 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.

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, who was not involved in the pentaquark search, said it was an “exciting and very convincing experiment­al breakthrou­gh” that would 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 existed in super- dense 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 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.

 ?? CERN ?? AN ARTIST’S conception of the pentaquark.
CERN AN ARTIST’S conception of the pentaquark.

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