Image courtesy of Brookhaven National Laboratory
A visual model of how particles emerging from heavy ion collisions flow from the point of impact depending on the viscosity of the matter produced. At low viscosity (left) the pattern of particles retains more of the detailed characteristics of the initial collision conditions than at high viscosity (right), where the details are blurred.
When gold or lead nuclei collide at high energies at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), the components of the nuclei, protons and neutrons, melt to form a hot soup of their constituent quarks and gluons known as the quark-gluon plasma (QGP), a form of matter thought to have existed shortly after the Big Bang. A new model that accurately describes the experimentally observed patterns of particles flowing out from the QGP suggests that the viscosity, or resistance to flow, is lower at RHIC than at the higher temperatures produced at the LHC. This finding defies intuition as conventional liquids, such as oil, flow more easily the hotter they get.
These results help answer questions about properties of matter that might have formed our early universe, for example, how close the viscosity of the QGP formed at RHIC comes to a limit derived from quantum mechanics, and how this resistance to flow varies as the temperature increases at the LHC. These findings will also help scientists better understand how dense concentrations of gluons known as a “color glass condensate” may affect the internal structure of heavy ions before they collide as well as the rapid transition into, and shape of, the liquid quark-gluon plasma.
When two gold or lead ions collide slightly off center, the regions that overlap have the approximate shape of an ellipse. The distribution of particles emerging from the collision—for example the number emerging horizontally vs. vertically transverse to the direction of the colliding beams—retains echoes of this initial shape. The shape of each collision is more complicated than an ellipse, and fluctuates from collision to collision, reflecting variations in the density of the particles in the lead or gold ions. The lower the viscosity of the hot soup or QGP the less the initial shape in the detected particle patterns is blurred. Detailed modeling of fluctuations of initial shapes, combined with viscous fluid flow calculations, allowed the scientists to extract what value of viscosity best agreed with the RHIC and LHC experimental data on finedetails of the patterns of particles flowing out of collisions. They found the extracted viscosity to be a factor of two smaller at RHIC than at the LHC as predicted for RHIC’s lower temperature.
Goldhaber Fellow, Nuclear Theory Group, Brookhaven National Laboratory
This research is funded by the Office of Nuclear Physics within the U.S. Department of Energy’s Office of Science.
C. Gale, S. Jeon, B. Schenke, P. Tribedy, R. Venugopalan, “Event-by-Event Anisotropic Flow in Heavy-ion Collisions from Combined Yang-Mills and Viscous Fluid Dynamics.” PRL 110 (2013) 012302.
Bjoern Schenke’s research page
Press release on discovery of perfect liquid at RHIC
A feature story with background on quark-gluon plasma, gluons, and color glass condensate
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