Duke theorists play role in search for superhot ’quark-gluon plasma’

Duke University theoreticians said their predictions helped guide the efforts of experimenters using Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) atom smasher to create an almost perfectly flowing fluid of hot, dense matter.

Duke physics professor Berndt Mueller and assistant physics professor Steffen A. Bass said that some of the experimental evidence would also support the idea that these collisions have recreated a state of matter — a quark-gluon plasma (QGP) –that last existed when the universe was only about a millionths of a second old. Their work is supported by the U.S. Department of Energy.

“The matter that they are seeing is even more interesting in some sense than we thought it would be,” Mueller said. “While the RHIC experimenters have taken a cautious stance by not explicitly declaring the discovery of a QGP, they have presented a compelling case for the achievement of an important milestone in the quest for the QGP.”

The Duke work is referenced in two of four papers by RHIC scientific collaborations that will be published simultaneously in the research journal Nuclear Physics A. Scientists from those collaborations will also describe their results at a 9 a.m. Monday, April 18 news conference in Tampa, Fla., to be held during the April meeting of the American Physical Society.

Brookhaven’s RHIC was designed to fleetingly re-create a quark-gluon plasma by simulating Big Bang conditions. It works by colliding gold atoms at extremely high energies.

It has been Mueller’s, Bass’s and fellow theoreticians’ jobs to predict how a QGP would signal its existence to the collider’s particle detectors.

Physicists believe all of today’s matter is locked in an inseparable embrace, with particles called quarks “glued” together as pairs and triplets by other particles called gluons. Protons, neutrons and other subatomic entities once considered fundamental are now thought to all be composed of quark-gluon composites.

Today quark-gluon embraces are inseparable in that the more quarks try to stray the stronger gluons draw them back together. However current physical theory describing quarks and gluons also makes an exception for the extreme conditions prevailing when the universe is thought to have begun in a cosmic fireball called the Big Bang.

Immediately after the Big Bang, quarks and gluons are theorized to have existed as separate entities in a superheated, charged fluid, the QGP.

By colliding heavy gold atoms, RHIC experimenters claim to have created temperatures up to 150,000 times hotter than the sun’s interior. Mueller and Bass said such temperatures should be hot enough to “melt” the very vacuum of space, like ice melts into water, creating unattached free quarks and gluons seemingly out of nothing.

Those artificially created particles are predicted to quickly rejoin and condense as the superhot plasma cools. But evidence for a QGP’s fleeting existence should remain, say the theorists, in the special types and behaviors of the particles that emerge.

In 1982, Mueller and colleague Johann Rafelski, now at the University of Arizona, predicted that the super-hot environment of a QGP would be the ideal breeding ground for the creation of an abundance of “strange” quarks.

These quark types are too heavy to exist today, except possibly under the most extreme conditions. Such extremes might include the interiors of superdense neutron stars that form after large normal stars run out of fuel and collapse.

Strange quarks created under the extreme conditions inside RHIC would quickly become unstable and decay into other particles. But the remnant particles would leave telltale evidence of what came before, Mueller said.

Confirming Mueller’s and Rafelski 23-year-old prediction, RHIC experimenters have detected evidence for the production of abundant strange quarks over the past three years. And RHIC’s detectors have also recorded several other signals for the evolution of a QGP that were more recently predicted by Duke theorists.

The Duke group considers one of its most exciting recent findings to be a novel approach for understanding how individual quarks and gluons in a QGP would recombine to form the particles observed by RHIC’s detectors.

Mueller, Bass and postdoctoral research associates Rainer Fries and Chiho Nonaka derived special sets of equations — called “scaling laws” — that would only hold true if those detected particles were made out of quarks and gluons that once circulated freely.

But the Duke theorists acknowledge that evidence for recombining particles is currently incomplete. “The particles that come out bear direct evidence of the previous behavior of quarks,” Mueller said. “But they don’t say anything about the gluons. So it’s hard to claim a QGP existed.”

Mueller and Bass said a quark-gluon plasma was previously expected to be a gas of weakly interacting particles. High pressure should have crushed this gas into a sticky, viscous fluid.

But instead, RHIC experimenters have observed an extremely low-viscosity fluid composed of strongly interacting particles. “The data indicate that it is the most perfect fluid we’ve ever seen,” Mueller said.

Said Bass, “If you want to think of a non-perfect fluid, think of honey. The gooeyness of honey is caused by its high viscosity. Creating a perfect fluid means the viscosity is very, very low. If you could drag a spoon through it would have almost no resistance.”

To deduce the dynamics of such an unexpected fluid, physicists have been using the mathematics of supersymmetry, an offshoot of string theory, the Duke theorists said. String theory posits that all matter is made of infinitesimal structures known as strings that adopt various modes of vibration to form all subatomic particles.

However, Bass suggested that, instead of string theory, his group’s recombinant scalar equations would provide a “much simpler” approach to describe the dynamics of such a perfect fluid. “Essentially the same equations that describe a drop of water could be used to understand the behavior of matter produced at RHIC,” Mueller added.

Bass and postdoctoral researcher Chiho Nonaka have spent the last three years developing an advanced “hydrodynamic” computer model that the Duke theorists said will put them in a leading position for research in this area.

Bass, an expert at creating computer models, said such calculations require following the complex interactions of particles moving in three dimensions at 30 to 50 percent of the speed of light. “There are not many people in the world who can do that,” he said.

For the last three years, their group has been using about half of a 64-computer cluster funded by the U.S. Department of Energy at Duke’s Department of Physics to run its RHIC-related models around the clock, Bass added. That cluster is now being upgraded with newer, faster machines.