The four experimental groups conducting research at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory have created a new state of hot, dense matter out of the quarks and gluons that are the basic particles of atomic nuclei, but it is a state quite different and even more remarkable than had been predicted. In an announcement at the April 2005 meeting of the American Physical Society in Tampa, Florida, and in peer-reviewed papers summarizing the first three years of RHIC findings, the scientists said that instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC’s heavy ion collisions appears to be more like a liquid.

The papers, which the four RHIC collaborations (BRAHMS, PHENIX, PHOBOS, and STAR) had been working on for nearly a year, were published simultaneously by the journal Nuclear Physics A8 and were also compiled in a special Brookhaven report.9 These summaries indicate that some of the observations at RHIC fit with the theoretical predictions for a quark-gluon plasma (QGP), the type of matter postulated to have existed just microseconds after the Big Bang. Indeed, many theorists have concluded that RHIC has already demonstrated the creation of quark-gluon plasma. However, all four collaborations note that there are discrepancies between the experimental data and early theoretical predictions based on simple models of quark-gluon plasma formation.

“We know that we’ve reached the temperature [up to 150,000 times hotter than the center of the sun] and energy density [energy per unit volume] predicted to be necessary for forming such a plasma,” said Sam Aronson, Brookhaven’s Associate Laboratory Director for High Energy and Nuclear Physics. But analysis of RHIC data from the start of operations in June 2000 through the 2003 physics run reveals that the matter formed in RHIC’s head-on collisions of gold ions (Figure 12) is more like a liquid than a gas.

That evidence comes from measurements of unexpected patterns in the trajectories taken by the thousands of particles produced in individual collisions. These measurements indicate that the primordial particles produced in the collisions tend to move collectively in response to variations of pressure across the volume formed by the colliding nuclei (Figure 13). Scientists refer to this phenomenon as “flow,” since it is analogous to the properties of fluid motion.

However, unlike ordinary liquids, in which individual molecules move about randomly, the hot matter formed at RHIC seems to move in a pattern that exhibits a high degree of coordination among the particles—somewhat like a school of fish that responds as one entity while moving through a changing environment.

Figure 12. Two views of one of the first full-energy collisions between gold ions at Brookhaven Lab's Relativistic Heavy Ion Collider, as captured by the Solenoidal Tracker At RHIC (STAR) detector. The tracks indicate the paths taken by thousands of subatomic particles produced in the collisions as they pass through the STAR Time Projection Chamber, a large, 3D digital camera.

Figure 13. These images contrast the degree of interaction and collective motion, or "flow," among quarks in the predicted gaseous quark-gluon plasma state (left) vs. the liquid state that has been observed in gold-gold collisions at RHIC (right). The "force lines" and collective motion in the observed collisions show the much higher degree of interaction and flow among the quarks in what is now being described as a nearly "perfect" liquid. An animation demonstrating the differences between the expected gas and the observed liquid can be viewed at http://real.bnl.gov/ramgen/bnl/RHIC_animation.rm.

“This is fluid motion that is nearly ‘perfect,’” Aronson said, meaning it can be explained by equations of hydrodynamics. These equations were developed to describe theoretically “perfect” fluids—those with extremely low viscosity and the ability to reach thermal equilibrium very rapidly due to the high degree of interaction among the particles. While RHIC scientists don’t have a direct measure of viscosity, they can infer from the flow pattern that, qualitatively, the viscosity is very low, approaching the quantum mechanical limit.

Together, these facts present a compelling case. “In fact, the degree of collective interaction, rapid thermalization, and extremely low viscosity of the matter being formed at RHIC make this the most nearly perfect liquid ever observed,” Aronson said.

In results reported earlier, other measurements at RHIC have shown “jets” of high-energy quarks and gluons being dramatically slowed down as they traverse the hot fireball produced in the collisions. This “jet quenching” demonstrates that the energy density in this new form of matter is extraordinarily high—much higher than can be explained by a medium consisting of ordinary nuclear matter.

“The current findings don’t rule out the possibility that this new state of matter is in fact a form of the quark-gluon plasma, just different from what had been theorized,” Aronson said. Many scientists believe this to be the case, and detailed measurements are now under way at RHIC to resolve this question.

Theoretical physicists, whose standard calculations cannot incorporate the strong coupling observed between the quarks and gluons at RHIC, are also revisiting some of their early models and predictions. To try to address these issues, they are running massive numerical simulations on some of the world’s most powerful computers. Others are attempting to incorporate quantitative measures of viscosity into the equations of motion for fluid moving at nearly the speed of light. One subset of calculations uses the methods of string theory to predict the viscosity of the liquid being created at RHIC and to explain some of the other surprising findings. Such studies will provide a more quantitative understanding of how “nearly perfect” the liquid is.

The unexpected findings also introduce a wide range of opportunity for new scientific discovery regarding the properties of matter at extremes of temperature and density previously inaccessible in a laboratory.

The STAR Collaboration, consisting of 614 researchers from 52 institutions in 12 countries, performs its data analysis on NERSC’s PDSF system. The computations carried out at NERSC are focused around analysis of the processed data, comparison of the data with experimental models, and studies of detector performance and acceptance. A storage resource allocation at NERSC supports an important collaboration between STAR, the Berkeley Lab Scientific Data Management Group, and the Particle Physics Data Grid (PPDG) project, involving development and deployment of data grid technology to move terabytes of data efficiently and securely.

One of STAR’s next priority goals at NERSC is to study the spin structure of the proton, focusing first on measurements of the gluon contribution.

Research funding (STAR only): NP, NSF, BMBF, IN2P3, RA, RPL, EMN, EPSRC, FAPESP, RMST, MEC, NNSFC, GACR, FOM, DAE, DST, CSIR, SNSF, PSCSR, STAA
Computational resources: NERSC, BNL
This article written by: Karen McNulty Walsh (Brookhaven National Laboratory), John Hules