LHC sees odd behavior in superhot particle soup

Coordinated motion observed in debris from lead-proton collisions

Strangely behaving subatomic particles at the world’s most powerful particle accelerator could lead to fresh insight into how matter behaves at the smallest scales and highest energies.

Ordinarily, the Large Hadron Collider near Geneva sends protons hurtling into each other at velocities approaching the speed of light. But for several hours in September, the machine collided protons into lead nuclei — tightly packed bundles of 82 protons and 126 neutrons. It was merely a test run, designed to calibrate instruments for future experiments.

But when physicists with the Compact Muon Solenoid collaboration analyzed the data, they quickly noticed that something was amiss. When a proton and lead nucleus collide, they shatter into smaller particles that jet out in all directions. The movement of each piece of shrapnel should be almost completely random; the direction of one particle should provide no clue to that of any other. Yet during these collisions, the particles’ directions tended to correspond to one other. Even particles located far from each other seemed to be coordinating their paths of travel.

“This is one of the most interesting unexpected effects we’ve observed at the LHC,” says Gunther Roland, a physicist at MIT and member of the CMS team. He and his team describe the odd behavior in an upcoming Physics Letters B. The team does not try to explain why the particles behaved as they did, but Roland notes that other physicists have come up with a few ideas.

One possibility is that the enormous heat and density in the collision zone actually liquefied the tiny bits of matter. Physicists first observed a liquidlike exotic state of matter, known as quark-gluon plasma, seven years ago when they slammed gold nuclei into each other at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York. Because quark-gluon plasma behaves like a liquid, it flows; the observed effect at the LHC could be the result of particles emerging from the outer edge of the plasma as it cools.

Physicists Raju Venugopalan and Kevin Dusling from Brookhaven present an intriguing alternative: The particles are actually influencing the movement of each other. If true, their theory would be yet another quirky manifestation of quantum mechanics, the strange laws of physics that reign at the smallest of scales. The Brookhaven physicists argue that gluons, the tiny particles that hold protons together, can form fields when moving near the speed of light. These fields allow gluons within multiple protons to interact and influence each other. In essence, Venugopalan and Dusling claim that under the right conditions, certain particles can stop behaving like individual pieces of matter and act more like a coordinated posse.

Berndt Mueller, a physicist at Duke University who is not on the CMS team, says he favors the quark-gluon plasma theory because it is the simplest explanation. But either explanation, if proven correct, would help physicists better understand the strange rules that dictate how matter works under the most extreme conditions. Physicists want to learn more about quark-gluon plasma because they believe all the matter in the universe was in this state a few millionths of a second after the Big Bang.

If Venugopalan and Dusling are right, then physicists would learn more about the structure of the proton and how it can influence the behavior of surrounding matter.

Both explanations do have holes. Roland notes that proton-lead collisions should not generate the extreme conditions necessary to produce quark-gluon plasma. And Venugopalan and Dusling’s prediction, while theoretically possible, has never been experimentally verified.

Fortunately, this mystery may find a quick resolution. Next month the LHC will again slam protons into lead nuclei, this time as part of a planned science-gathering run of several weeks. That should provide the team with up to 100,000 times more data to determine which explanation holds up.

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