Smashing debut for a subatomic fireball.

Smashing debut for a subatomic fireball

Amillionth of a second after its creation in the Big Bang, the universe was so hot that neutrons and protons had not yet condensed out of a sea of quarks and gluons. Now researchers have obtained experimental evidence supporting this scenario. They found the evidence for a new state of matter -- a quark-gluon plasma -- in the spray of subatomic particles released by a fireball created in a violent head-on collision between two atomic nuclei.

Protons and neutrons, the two constituents of atomic nuclei, are each thought to consist of three quarks, bound together by force-carrying particles known as gluons. Theoretical calculations predict that at sufficiently high temperatures, protons and neutrons overlap and lose their identities. Collections of these particles would "melt" into a losse agglomeration of quarks and gluons. Although still confined, the loosely bound quarks would be free to roam over distances much larger than the size of a neutron or proton. Then, as the material cooled, quarks would begin to recombine, producing particles such as pions, which consist of pairs of quarks.

To generate the high energies needed to create a quark-gluon plasma, physicits at CERN, the European Laboratory for Particle Physics near Geneva, Switzerland, use an accelerator to hurl the nuclei of oxygen atoms, each a bundle of eight protons and eight neutrons, at a gold target. Like a high-speed bullet hitting a tin can, an oxygen nucleus traveling at nearly the speed of light punches a neat hole in a gold nucleus excising about 50 of the gold's 197 neutrons and protons. The oxygen, carrying an energy of 3.2 trillion electron-volts, deposits a significant proportion of this energy into the excised material, creating the atomic equivalent of a fireball.

"You're creating a mini-Big Bang," says Thomas J. Humanic of the University of Pittsburgh. "You're simulating the early universe, a microsecond after creation." Humanic was a member of the large CERN team that performed the experiment and analyzed the results.

The researchers determine the fireball's nature and geometry by studying how quickly it emits pairs of negative pions as it cools. "It survives an unusually long time, longer than one would expect," says theorist George F. Bertsch of Michigan State University in East Lansing. "It takes a long time for the plasma to decay back to ordinary matter."

The fireball is also much larger in size than it would be if it consisted only of protons and neutrons knocking about. Both the bigger size and the longer lifetime indicate that during a collision, at least some of the fireball's protons and neutrons undergo a phase transition, or melt, to form a droplet of quark-gluon plasma.

"Unambiguous signatures for this predicted change of state have been hard to find," Bertsch says. "This is the first physical indication of the new state of matter that people have been looking for." The discovery was announced last week in Lenox, Mass., at the Quark Matter '88 meeting.

"It's a preliminary result, but it's very tantalizing," says Humanic. "And it seems to agree very well with theoretical ideas." The CERN researchers have so far observed 105 suitable oxygen-gold collisions. They want to increase the number of observations and to test other nuclei. Alternative strategies include firing sulfur nuclei at a sulfur target and lead nuclei against a lead target to study whether the type of nucleus involved in a collision has an effect on plasma formation.

If the quark-gluon plasma state is confirmed, the finding will encourage further theoretical work, Bertsch says. It would be interesting to determine the details of how a plasma droplet gets rid of its energy and how the system achieves equilibrium. In addition, the surface of a quark-gluon plasma droplet may have unusual features.