SCIENCE

The Large Hadron Collider shut down its proton beams on Nov. 4, 2010, and quickly began circulating beams of lead ions, a run scheduled to last a month. Within days, the first results from ALICE, the LHC experiment designed specifically to study heavy-ion collisions, were posted online. Two weeks after the start of the lead-lead run, CERN’s press office announced what it called “new insight into the primordial universe” from three LHC experiments, ALICE plus ATLAS and CMS. The latter are broad-coverage detectors which also have programs for investigating heavy-ion collisions. Finally the ALICE collaboration posted two more results, shortly before the LHC ended its lead-ion run on Dec. 6.

Peter Jacobs of the Nuclear Science Division (NSD) at Berkeley Lab, which hosts U.S. participation in ALICE, said the experience was both exciting and exhausting: “ALICE generated four key papers in two weeks.” The frenzy is likely to repeat itself a year from now, after a return to circulating proton beams at the LHC to resume in February. The next heavy-ion run is scheduled to begin in late November, 2011.

A glimpse of the early universe

The raison d’être of high-energy, heavy-ion collisions is to recreate the quark-gluon plasma, or QGP, a hot soup of elementary particles that existed a few millionths of a second after the big bang. Collisions of lone protons, which consist of three quarks and the gluons that tie them together, can’t reconstruct the QGP.

By contrast, the nucleus of a lead atom is a dense collection of 82 protons and 126 neutrons. When two lead nuclei collide with enough added energy, the resulting extreme heat “deconfines” their many quarks and gluons, freeing them to mingle and expand until they quickly condense into new collections of particles whose paths can be tracked.

Sometimes the lead nuclei crash head-on, but more often the collisions are off-center. One of the first ALICE papers reported the number of charged particles produced from head-on impacts – more than twice as many as produced in collisions of gold ions at RHIC, Brookhaven’s Relativistic Heavy Ion Collider; this result disagrees with a number of theories predicting how particle production would scale with collision energy. The other initial ALICE paper discussed the “elliptic flow” that results from off-center collisions, a sign of the quark-gluon plasma.

“Right now with ALICE we are running through the same kinds of measurements we did at RHIC,” says Jacobs. “The difference is that the lead-lead collisions are at 2.76 TeV instead of RHIC’s 200 GeV” – that is, 2.76 trillion electron volts instead of 200 billion electron volts, an almost 14-fold increase in energy. Based on the increase in energy and the experience gained at RHIC, says Jacobs, “in just a few weeks we’re doing what took several years at RHIC.”

Scientists with Berkeley Lab’s Nuclear Science Division have played a major role not only in the present work at the LHC – NSD’s Mateusz Ploskoń is coordinator of ALICE’s current run, and NSD’s Constantin Loizides was a leading author of two papers on particle production – but they also made many of the theoretical and experimental discoveries underlying major aspects of the entire quark-gluon plasma research program.