March 2, 2000

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The announcement out of the European Center for Nuclear Research (CERN) that a new form of matter called a "quark-gluon plasma" (QGP) may have been created is another important milestone in a series of studies that began in the early 1980s at the U.S. Department of Energy's Lawrence Berkeley National Laboratory and will continue with a new round of experiments starting later this year on the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. From Berkeley, to Geneva, and on to Long Island, researchers with Berkeley Lab's Nuclear Science Division (NSD) have been key players in the search for the elusive plasma believed to have been the antecedent of all matter in the universe today.


Quarks are one of the families of fermions, the basic constituents of matter. Gluons are bosons, carriers of the strong force that bind quarks together into hadrons such as protons or neutrons. In the ordinary matter that makes up us and the world in which we live, quarks are never free of other quarks or gluons. In the experiments at CERN's Super Proton Synchrotron (SPS), however, collisions between high-energy beams of lead nuclei generated temperatures 100,000 times hotter than the interior of the sun. Within the extremely dense fireball at the heart of these collisions, the ties that bind quarks and gluons may have melted, creating a soup-like plasma of free-floating individual particles.

QGP is believed to have been the state of matter under the extreme pressure and temperature conditions that prevailed in the first 10 microseconds after the Big Bang. Though highly transient -- a QGP quickly cools and reverts to the ordinary state of matter -- the QGP in its brief existence, set the stage for the combinations of particles that make up our universe today. QGP is also thought to be the state of matter in the dense cores of neutron stars. Creating a QGP in particle accelerators could yield new insights into how our universe was formed and a better understanding of the behavior of atomic nuclei.

"A common assessment of the collected data leads us to conclude that we now have compelling evidence that a new state of matter has been created at energy densities that have never been reached over appreciable volumes in laboratory experiments before and which exceed by more than a factor of 20 that of normal matter," the CERN announcement read.

In response to this announcement, Xin-Nian Wang, a theorist with NSD says, "The experiments at CERN so far are excellent and have provided us much more information that we did not know before. However, like so many other (relativistic heavy ion) experiments, for each QGP signal there are backgrounds. Sorting out these backgrounds is a challenge. It is like a murder trial without a smoking gun. The key is proof beyond a reasonable doubt. The experiments at SPS have failed to prove beyond a reasonable doubt that the phenomena can only be attributed to QGP and nothing else."

Nuclear theorists are in agreement that if atomic nuclei are squeezed hard enough under conditions of high pressure and temperature, a QGP will form. Over the course of several years worth of experiments at the SPS, nuclei of lead, with their 208 hadrons (82 protons and 126 neutrons), were accelerated into beams with energies in excess of 160 billion electron volts per nucleon and smashed together.

While the data suggests that these lead nuclei were squeezed hard enough to have produced de-confined quark-gluon matter (also referred to as "partonic" matter), many nuclear physicists at Berkeley Lab and elsewhere do not believe there is enough evidence that a QGP was produced.

Everyone, including scientists at CERN, agrees that more definitive proof should be forthcoming from the experiments at RHIC where collisions of gold nuclei will take place at 10 times higher energy densities than the lead nuclei at the SPS. These experiments are expected to yield a true QGP and hold that state long enough for it to be studied.

Berkeley Lab researchers designed and constructed a large volume Time Projection Chamber and a significant portion of the electronics for one of RHIC's two large-scale detector systems, STAR. The higher energy densities in combination with a sophisticated detector array like STAR should make it possible to produce a QGP and perform the types of systematic and meticulous studies needed to understand it.

For example, says Wang, "One way to detect QGP is to scan over a wide range of reactions. Since the matter produced in high-energy heavy-ion collisions is in very small amounts, one does not expect to see very sharp features or discontinuity in the scan. However, if one finds a bump or a step in the scan, it would be an unambiguous signature of QGP."

Another of the promising new experiments scheduled for RHIC is one in which "jets" (energetic beams of quarks) will be observed crossing through the center of the collision fireball where the QGP would be. Analyzing how the jets propagate through the fireball and measuring the amount of "quenching" or energy loss that occurs should reveal whether or not a QGP was created.

Hans Georg Ritter is one of the pioneers in the QGP hunt and now heads NSD's Relativistic Nuclear Collisions program. He says, "The combined results from the experiments at CERN present tantalizing hints of the exciting new physics that await us at RHIC."

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