Melting nuclei re-create Big Bang broth

A powerful particle accelerator in Switzerland may have briefly reproduced an ancient state of matter that pervaded the universe in the first microseconds after its birth, researchers have announced.

Nuclear shrapnel: More than 1,000 particles streak away from the collision of two lead nuclei at CERN’s SPS accelerator. A computer reconstructed the trajectories from detector readings. CERN Geneva

If confirmed, the findings from the European Laboratory for Particle Physics (CERN) in Geneva indicate that scientists have glimpsed a substance governed by titanic effects called color forces.

Researchers are eager to study properties of the bizarre material, which may also exist inside collapsed stars known as neutron stars.

CERN investigators smashed lead nuclei flying at nearly light’s speed into other nuclei in fixed targets. The collisions produced fireballs 100,000 times hotter than the sun’s core and 20 times the density of an atom’s nucleus. In such microfurnaces, theorists propose, protons and neutrons may dissolve and momentarily set free a furious swarm, or plasma, of quarks and gluons. Quarks possess a characteristic that physicists call color, which is loosely analogous to electric charge. Under normal conditions the potent color force keeps quarks and gluons tightly confined within the nuclear particles.

Researchers have sought the quark-gluon plasma since at least the mid-1980s (SN: 10/8/88, p. 229). In their Feb. 10 announcement, the CERN teams reported finding traces perhaps not of the plasma itself—which has a very narrow scientific definition—but of something closely akin to it.

“All that we know is that we have evidence for a state in which quarks and gluons are deconfined,” says Federico Antinori of the Instituto Nazionale di Fisica Nucleare in Padova, Italy. Since 1994, seven separate CERN teams have studied lead-lead and lead-gold collisions at the laboratory’s Super Proton Synchrotron (SPS) accelerator. Some of those groups have previously reported findings that also hinted, although less strongly, at the quark-gluon plasma (SN: 9/21/96, p. 190: For the latest announcement, all the teams put their most up-to-date findings together like pieces of a puzzle, Antinori says. “Some individual signals may be controversial, but when you fit the picture together, the evidence is compelling,” he argues.

“All this agrees with what would be expected,” concurs Johann Rafelski at the University of Arizona in Tucson. “CERN has done great, significant work.” Some physicists, however, remain unconvinced. “The important thing is the evidence for deconfined matter. I don’t feel that it is compelling,” comments James Nagle of Columbia University. Although he says the SPS research is of high quality, Nagle contends that many other physicists share his skepticism.

The timing of the announcement has also raised eyebrows. Next month, the new Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in Upton, N.Y., begins its much-heralded search for the quark-gluon plasma.

CERN reports measurements indicating that SPS packed sufficient wallop to make a quark-gluon plasma. Other signs, also indirect, include anomalous abundances of certain quark types within particles formed when fireballs cooled. When CERN scientists combed their data for direct evidence, however, such as the gamma rays that physicists expect such plasma to emit, the signals they identified were unconvincing.

Stronger signs could show up soon at the new collider, where nuclei will smash together with 10 times greater energy than at SPS, scientists say. Although disappointed that CERN may have beaten the Brookhaven facility to the punch, RHIC director Satoshi Ozaki welcomed the findings as “good news.” Assuming they are correct, “we are now sure we can study [the new state of matter] in detail and establish what it is,” he says.

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