Jul 7, 2017

CERN Physicists Find a Particle With a Double Dose of Charm

Physicists have discovered a particle that is doubly charming.


Researchers reported on Thursday that in debris flying out from the collisions of protons at the CERN particle physics laboratory outside Geneva, they had spotted a particle that has long been predicted but not detected until now.

The new particle, awkwardly known as Xi-cc++ (pronounced ka-sigh-see-see-plus-plus), could provide new insight into how tiny, whimsically named particles known as quarks, the building blocks of protons and neutrons, interact with each other.

Protons and neutrons, which account for the bulk of ordinary matter, are made of two types of quarks: up and down. A proton consists of two up quarks and one down quark, while a neutron contains one up quark and two down quarks. These triplets of quarks are known as baryons.

There are also heavier quarks with even quirkier names — strange, charm, top, bottom — and baryons containing permutations of heavier quarks also exist.

An experiment at CERN, within the behemoth Large Hadron Collider, counted more 300 Xi-cc++ baryons, each consisting of two heavy charm quarks and one up quark.

The discovery fits with the Standard Model, the prevailing understanding of how the smallest bits of the universe behave, and does not seem to point to new physics. "The existence of these particles has been predicted by the Standard Model," said Patrick Spradlin, a physicist at the University of Glasgow who led the research. "Their properties have also been predicted."

Dr. Spradlin presented the findings on Thursday at a European Physical Society conference in Venice, and a paper describing them has been submitted to the journal Physical Review Letters.

Up and down quarks have almost the same mass, so in protons and neutrons, the three quarks swirl around each other in an almost uniform pattern. In the new particle, the up quark circulates around the two heavy charm quarks at the center. "You get something far more like an atom," Dr. Spradlin said.

Quark interactions are complex and difficult to calculate, and the structure of the new particles will enable physicists to check the assumptions and approximations they use in their calculations. "It's a new regime in quark-quark dynamics," said Jonathan L. Rosner, a retired theoretical physicist at the University of Chicago.

The mass of the Xi-cc++ is about 3.8 times that of a proton. The particle is not stable. Dr. Spradlin said the scientists had not yet figured out its lifetime precisely, but it falls apart after somewhere between 50 millionths of a billionth of a second and 1,000 millionths of a billionth of a second.

For Dr. Rosner, the CERN results appear to match predictions that he and Marek Karliner of Tel Aviv University made.

What is less clear is how the new particle fits in with findings from 2002, when physicists working at Fermilab outside Chicago made the first claim of a doubly charmed baryon, one consisting of two charm quarks plus a down quark (instead of the up quark seen in the CERN experiment).

The two baryons should be very close in mass, but the Fermilab one was markedly lighter than what the CERN researchers found for Xi-cc++, and it appeared to decay instantaneously, in less than 30 millionths of a billionth of a second.

Theorists like Dr. Rosner had difficulty explaining the behavior of the Fermilab particle within the Standard Model. "I didn't have an honest alternative to allow me to believe that result," he said.

Peter S. Cooper, a deputy spokesman for the Fermilab experiment, congratulated the CERN researchers on their discovery. "That paper smells sweet," he said. "From an experimental point of view, there's nothing wrong. They definitely have something."

But he said the Fermilab findings still stood, too. He acknowledged that the two results do not readily make sense together.

"I consider this a problem for my theoretical brethren to work out," Dr. Cooper said. He added that it was a textbook example of the scientific method: "Our theoretical colleagues make a prediction. We go out and make a measurement and see if it's right. If it isn't, they go back and think harder."

It is possible one of the experiments is wrong. Researchers at other laboratories, including at CERN, have sought to detect the Fermilab baryon without success. Dr. Spradlin said he and his colleagues are searching the same data that revealed the Xi-cc++ for the baryon with two charm quarks and one down quark. That could confirm the Fermilab findings or reveal a mass closer to theorists' expectations.

"We should be able to see it with the data we have," Dr. Spradlin said. "I think we are very close to resolving this controversy."

The New York Times