CERN, the European laboratory for particle physics, located in Switzerland, has just made public results of its five-year program of heavy-ion collision experiments that hint at the formation of a quark-gluon plasma. Has CERN scooped the Relativistic Heavy Ion Collider (RHIC), which is expected to begin heavy-ion experiments at the U.S. Department of Energy's Brookhaven National Laboratory this year? The answer is no.
CERN states that their evidence is "circumstantial." But their results do generate confidence that a quark-gluon plasma may be directly observed for the first time when RHIC achieves full operational status.
The experiments at CERN are the highest-energy, man-made, heavy-ion collisions yet achieved. A similar round of experiments at somewhat lower energy was carried out with gold ion beams at Brookhaven's Alternating Gradient Synchrotron (AGS) in 1993. The temperatures and densities in these CERN and AGS collisions approach the extreme values at which theories predict quark-gluon plasma may form. And many of the observations from the seven experiments at CERN are consistent with the predicted signatures of a quark-gluon plasma.
Analyzed independently, none of the CERN experiments is definitive. But taken together, they provide circumstantial evidence that quark-gluon plasma may have been produced.
Direct measurements, such as detecting electromagnetic signals from a quark-gluon plasma, will only be possible at the much higher collision energies that will be achieved at RHIC and, later, at CERN's Large Hadron Collider. The RHIC experiments will be more than ten times more powerful than those at CERN. The higher temperatures achieved in RHIC's collisions will allow the quark-gluon plasma to linger long enough for the kinds of direct observations that are not possible with the CERN experiments.
The CERN results are quite encouraging, says Tom Ludlam, Brookhaven's Deputy Associate Director for High-Energy and Nuclear Physics. "These results set the stage for the definitive round of experiments at RHIC in which the quark-gluon plasma will be directly observed, opening up a vast landscape for discovery regarding the nature and origins of matter."
Brookhaven's Director John Marburger congratulated CERN scientists on their achievement, stating that "piecing together even this indirect evidence of the quark-gluon plasma is a tour de force. The CERN teams have pressed their capabilities to the limit to extract these tantalizing glimpses into a new domain of matter."
For CERN's announcement, see http://www.cern.ch/CERN/Announcements/2000/NewStateMatter.
The U.S. Department of Energy's Brookhaven National Laboratory creates and operates major facilities available to university, industrial and government personnel for basic and applied research in the physical, biomedical and environmental sciences, and in selected energy technologies. The Laboratory is operated by Brookhaven Science Associates, a not-for-profit research management company, under contract with the U.S. Department of Energy.
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