RHIC Detects Liquid State of Quark-Gluon Matter

By Ernie Tretkoff

Analysis of the weird quark-gluon matter produced at RHIC shows that the substance is more like a liquid than a gas, researchers reported at the APS April Meeting. The researchers from the Relativistic Heavy Ion Collider at Brookhaven National Laboratory announced the results of recent analysis of the quark gluon matter they have been producing for years–a state many scientists expected would be the "quark gluon plasma."

"Theorists expected this phase to exist. The properties of this phase are surprising. The big surprise is that it’s a liquid," said Brookhaven theorist Dmitri Kharzeev.

The RHIC collaborations made this announcement at a press conference during the APS April Meeting in Tampa. They will also publish a set of papers in the journal Nuclear Physics A.

Researchers from all four RHIC collaborations–PHENIX, STAR, PHOBOS, and BRAHMS–participated in the announcement. The new results are based on analysis of data from the 2000-2003 run.

RHIC creates the blob of quark-gluon matter by smashing gold nuclei together at very high energies. Under these extreme conditions, the quarks and gluons normally bound in nucleons can become unbound. The quark gluon matter is extremely hot and dense, nearly 150,000 times as hot as the sun’s core and 100 times the density of a nucleus, said the researchers. The blob lasts for only about 10^-23 seconds.

By analyzing the distribution of particles that spray out of the quark-gluon blob as the state decays, researchers from the four collaborations have concluded that this strange, short-lived state behaves like a liquid, rather than a gas, as some physicists had predicted. In this state, the quarks and gluons interact with each other more strongly than expected, and flow together with very little viscosity, like a nearly perfect fluid. "It’s more fluid than the water in this glass," said Kharzeev, pointing to the glass in front of him.

Some theorists have been saying for several years that this quark gluon matter produced by RHIC is the much sought-after quark-gluon plasma, a state that is thought to have existed briefly in the very early universe. "Circumstantially, that fits the data," said Sam Aronson, Associate Director for High Energy and Nuclear Physics at Brookhaven. "Every physicist has his own take on whether this is the quark-gluon plasma. I think it is."

But the RHIC scientists have cautiously avoided making an official announcement of the quark-gluon plasma. They said at the press conference in Tampa that they are more concerned with studying the properties of this bizarre state of matter than with naming it.

Some researchers believe that the quark-gluon plasma would have filled the universe in the first microseconds after the big bang, and therefore the RHIC studies could help provide insight into the extreme conditions in the early universe. "We think we’re looking at a phenomenon last seen in the universe more than 13 billion years ago," said Aronson.

However, Kharzeev said, "The findings are so new no one has looked at implications for cosmology."

RHIC researchers plan to continue to study the properties of this unusual state of matter by measuring its heat capacity, viscosity, and temperature.

"There are lots of exciting questions. We’re at the edge of new terrain. We need to go explore it," said Aronson.

However, due to budget cuts, RHIC will have to decrease its operating time from 30 to 12 weeks per year. "This clearly slows down the productivity of the program," said Aronson.