Physicists have discovered a particle that is doubly charming. Researchers recently reported that in debris flying out from the collisions of protons at the European Organisation for Nuclear Research (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.
Complex interactions
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, Scotland who led the research. “Their properties have also been predicted.”
Patrick presented the findings at a recent 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 centre. “You get something far more like an atom,” Patrick 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, USA. The mass of the Xi-cc++ is about 3.8 times that of a proton. The particle is not stable. Patrick 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 Jonathan, the CERN results appear to match predictions that he and Marek Karliner of Tel Aviv University, Israel made. What is less clear is how the new particle fits in with findings from 2002, when physicists working at Fermilab outside Chicago, USA made the first claim of a doubly charmed baryon, one consisting of two charm quarks plus a down quark. 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 Jonathan had difficulty explaining the behaviour 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.”
Confirming results
But he said the Fermilab findings still stood, too. He acknowledged that the two results do not readily make sense together. “I consider this as a problem for my theoretical brethren to work out,” Peter 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 that one of the experiments is wrong. Researchers at other laboratories, including at CERN, have sought to detect the Fermilab baryon without success. Patrick 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,” Patrick said. “I think we are very close to resolving this controversy.”