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May 13, 2005
RHIC Springs a Surprise
When the Universe was More Like a Liquid Than a Gas

"In the end, it's not what you expected to find but what you actually found that's important," said Hans Georg Ritter, a physicist with Berkeley Lab's Nuclear Science Division.

A collision of gold nuclei in the STAR experiment at RHIC creates a fireball of pure energy from which thousands of new particles are born. By tracking the particles, here represented by threadlike lines of color, through STAR's Time Projection Chamber (designed and built at Berkeley Lab) the properties of the fireball can be reconstructed.

Ritter was commenting on the April, 2005 announcement from Brookhaven National Laboratory (BNL) about reports on three years' worth of data from the four detector groups at BNL's Relativistic Heavy Ion Collider (RHIC), a $600-million particle accelerator that has been used to create the hottest, densest matter ever observed.

RHIC's experimental groups have been searching for a gaseous, ephemeral state of matter called a quark-gluon plasma, which is believed to have existed in the first few microseconds after the universe was born. Instead, they now report, in recreating the superhot, superdense conditions of the universe immediately following the Big Bang they have observed a state of matter that acts more like a liquid than a gas — a finding that was actually foreshadowed by experimental results in the 1980s at Berkeley Lab.

RHIC is the world's largest facility for nuclear physics research. Since it began operations in the summer of 2000, RHIC has been used to generate high-energy (100 billion electron volts per nucleon), head-on collisions, either between two beams of gold nuclei or a beam of gold nuclei and a beam of deuterons ("heavy hydrogen" nuclei consisting of one proton and one neutron). Temperatures within the nuclear fireballs produced in these collisions approach one trillion degrees above absolute zero, which is about 300 million times hotter than the surface of the sun.

It was predicted that such temperatures would be hot enough to "melt" the gold nuclei into their constituent quarks and gluons and allow these particles to briefly exist free of one another in a gas. Quarks are one of the basic constituents of matter. Gluons are carriers of the strong force that binds triplets of quarks together into protons or neutrons. In the ordinary matter that makes up the world in which we live, quarks are never free of other quarks or gluons.

In a set of four white papers presented in a special edition of the journal Nuclear Physics A the four RHIC detector groups, BRAHMS, PHENIX, PHOBOS, and STAR, report on the results of data gathered between 2000 and 2003. These results, based on millions of collisions, point to the observation of a new state of matter in which quarks and gluons are strongly coupled and exhibit a fluid motion that is nearly perfect — meaning its viscosity is close to zero, or friction free. This phenomenon, in which quarks and gluons move together in a highly coordinated manner in response to variations of pressure across the volume formed by the colliding nuclei, is called "collective flow."

Force lines indicate the much higher degree of interaction and flow among quarks in the liquid state (left), as observed in collisions of nuclei at RHIC, than in the gaseous quark-gluon plasma state which had been expected (right).

The first observations of collective flow were recorded in 1984 at Berkeley Lab's now-defunct Bevatron (actually the Bevelac — the accelerator configuration resulting from coupling the Bevatron, a synchrotron, to a linear accelerator called the SuperHILAC). These observations provided the first evidence that a particle accelerator could be used to compress nuclear matter to a state of high temperature and density, paving the way for the construction of RHIC and the search for the quark-gluon plasma.

"Much of the basic theoretical and experimental framework for our understanding of collisions between heavy nuclei was developed at the Bevalac," Ritter has said. "The collective flow we observed in 1984 gave us our initial look at how dense nuclear matter behaves."

Ritter was a physicist with GSI, the Association for Heavy Ion Research in Darmstadt, Germany, which conducted those original collective-flow experiments at the Bevelac in collaboration with scientists at Berkeley Lab. Today he heads Berkeley Lab's Relativistic Nuclear Collisions (RNC) group, whose members are participants in the STAR experiments. (STAR stands for Solenoidal Tracker At RHIC.) It was data from STAR, in an analysis led by Berkeley Lab physicist Art Poskanzer, that provided the first observation of collective flow at RHIC — which is much stronger than the collective flow observed at the Bevalac.

Ritter says that while the findings being reported in the four white papers are remarkable and critically important, they do not surprise him. "The experiments that we are conducting at RHIC are aimed at producing a new state of matter, which must be done through strong interactions among constituents," Ritter said. "It should not be a surprise, then, that the matter we observe is strongly interacting."

Xin-Nian Wang and Hans Georg Ritter are physicists in Berkeley Lab's Nuclear Sciences Division. (Photo Roy Kaltschmidt)

Xin-Nian Wang, a physicist who heads Berkeley Lab's Nuclear Theory group, believes the near-perfect liquid state of matter that's been observed at RHIC is a surprise — but in retrospect it should not have been.

"Theorists focused on the idea of studying the quark-gluon plasma as if it were a weakly interacting gas because such a system is easier to calculate than a strongly couple system or a liquid," Wang said.

Wang and other theorists involved with the RHIC experiments have noted that the equations developed in recent years to explain string theory — the idea that the fundamental properties of the universe are contained within 10 dimensions, rather than in three spatial dimensions plus time — might also be applied to understanding the strongly interacting properties of the quark-gluon liquid that's been found at RHIC.

"This is how we learn," Wang said. "We observe something new, and then we look for the best explanation of what we have observed. This might well lead us to something new and even more exciting. That's what science research is all about."

The quark-gluon liquid created from colliding nuclei at RHIC is dense, but like a perfect fluid it flows freely.

Wang and Ritter agree that the theorists will need a much more detailed experimental characterization of the quark-gluon liquid state. While the quark-gluon plasma is thought to have survived for only about the first 10 millionths of a second after the Big Bang, before rapidly cooling temperature condensed it into ordinary matter, it set the stage for creating the particles that make up our universe today. A better understanding of the exotic states of matter being created at RHIC might yield new insights into how our universe was formed.

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