SCIENCE

The protons and neutrons that constitute the nuclei of atoms are made of quarks bound by gluons (foreground, left). In a hot, dense quark-gluon plasma (background), quarks and gluons are unbound and free to move independently.

How flow reveals the QGP

Berkeley Lab’s Art Poskanzer is the discoverer of the phenomenon of collective flow in high-density, high-temperature nuclear matter. Poskanzer was a cofounder of the STAR collaboration at RHIC and co-author of STAR’s first paper, which announced the discovery of elliptic flow at collider energies.

Elliptic flow is characteristic of off-center collision events energetic enough to create the quark-gluon plasma and was a major source of information about the characteristics of the quark-gluon plasma at RHIC. Indeed, it underlay the surprising 2005 announcement that RHIC’s heavy-ion collisions were generating a near-perfect fluid.

“The elliptic flow ALICE measured at the LHC is an increase of about 30 percent over what we saw at RHIC,” says Jacobs. “Elliptic flow tells us about the viscosity of the quark-gluon plasma. The flow we measured at RHIC was well represented by a theoretical model that said the viscosity was small, but we’ll have to wait until the theorists mull over the ALICE result before we know what it’s telling us about viscosity.”

What viscosity reveals about a liquid is how strongly its particles interact. Although it sounds counterintuitive, the more strongly the particles interact, the lower the viscosity.

“Viscosity isn’t the same as stickiness, it’s a fluid’s resistance to flow,” Jacobs explains. “If you imagine a liquid in which the particles don’t interact, you have a liquid that doesn’t flow at all – push on it with a piston and the impulse just moves right through without entraining the other particles. By contrast, in a perfect liquid the particles interact strongly. Push it with a piston and the entire fluid flows. It has no resistance to flow, thus zero viscosity.”

The degree of viscosity doesn’t come directly out of the experiments performed so far at ALICE, however, and the connection between the observed flow and the interaction of the particles is yet to be determined.

“If we’re correct, the latest observations show the QGP viscosity dropping,” Jacobs says. “String theorists predict a lower bound on the viscosity of any fluid. While the connection between our measurements and string theory is highly speculative, in heavy-ion collisions we seem to be approaching that limit. RHIC didn’t achieve it. Maybe the LHC will.”