In 1984, a collaboration of researchers with Berkeley Lab and from GSI in Germany, working at the Bevalac -- the accelerator configuration when the Bevatron was coupled to the SuperHILAC -- found the first direct evidence that nuclear matter can be compressed to high temperature and density in an accelerator. This marked the first major milestone in the search for a quark-gluon plasma or QGP.

THE BEVALAC IN WHICH IONS OF HEAVY ELEMENTS, CREATED IN THE SUPERHILAC (TOP), WERE SENT DOWN A BEAM PIPE FOR FURTHER ACCELERATION AND EXPERIMENTAL WORK IN THE BEVATRON

The evidence discovered was a phenomenon called "collective flow." In the immediate aftermath of collisions between a beam of niobium and a niobium target, it was observed that the protons, neutrons and other subatomic particles released in these collisions hung together for a fraction of a second before flying apart.

"This was our initial look at how dense nuclear matter behaves," says Art Poskanzer, a physicist with Berkeley Lab's Nuclear Science Division (NSD) and one of the leaders of that collective flow experiment.

Another participant in that collective flow experiment was physicist Hans Georg Ritter who at the time was with GSI and now heads NSD's Relativistic Nuclear Collisions program. Says Ritter, "Much of the basic theoretical and experimental framework for our understanding of collisions between heavy nuclei was developed at the Bevalac."

Within a few years, the Berkeley Lab researchers had shifted their focus to the Super Proton Synchrotron (SPS) at CERN where collision energies were 100 times higher than could be reached at the Bevalac. In collaboration with scientists from GSI and CERN, they initiated a relativistic heavy ion program. Lab researchers helped construct an injector that initially enabled the SPS to accelerate oxygen and sulfur to relativistic energies, and eventually to produce the lead-lead collisions whose results are now being reported.

In a head-on collision between two beams of lead nuclei in the SPS, about 2,500 particles are created, virtually all of them hadrons. Since a change-of-state from ordinary matter to a QGP is almost immediately followed by a change back to ordinary matter, the plasma's existence must be inferred by studying its final products. This requires an elaborate system of detectors.

Berkeley Lab researchers played a major part in the construction of several critical detector systems for the SPS heavy ion program. They also played a major part in the science done on those detectors.

The tradition of Berkeley Lab contributions to detectors aimed at capturing the elusive QGP has continued with the new facility at Brookhaven called the Relativistic Heavy Ion Collider or RHIC. Berkeley Lab scientists and engineers led the collaboration that designed and constructed STAR -- the Solenoid Tracker At RHIC -- one of the accelerator's two giant detector systems. They will be major participants in the science there as well.

Additional Information:

• Quest for quark gluon plasma heats up
• Slide show: STAR and the quest for the quark-gluon plasma