Research Confirms Collective Nature of Quark Soup’s Radial Expansion

This is an example of a heavy-ion collision event recorded by ATLAS in November 2015. Tracks reconstructed from hits in the inner tracking detector are shown as orange arcs curving in the solenoidal magnetic field. The green and yellow bars indicate energy deposits in the Liquid Argon and Scintillating Tile calorimeters respectively. (ATLAS Collaboration)

Scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory and Stony Brook University played leading roles in the analysis of heavy ion collisions at the Large Hadron Collider (LHC) that provide evidence that a pattern of "flow" observed in particles streaming from these collisions reflects those particles' collective behavior.

LHC is the world's most powerful particle collider, located at CERN, the European Organization for Nuclear Research, and the measurements reveal how the distribution of particles is driven by pressure gradients generated by the extreme conditions in these collisions, which mimic what the universe was like just after the Big Bang.

The research is described in a paper published in Physical Review Letters by the ATLAS Collaboration at the LHC.

Jiangyong Jia, left, led and oversaw research on radial flow by his graduate student, Somadutta Bhatta, right.

The international team used data from the LHC's ATLAS experiment to analyze how particles flow outward in radial directions when two beams of lead ions - lead atoms stripped of their electrons - collide after circulating around the 17-mile circumference of the LHC at close to the speed of light.

"Earlier measurements revealing that particles flow collectively from heavy ion collisions were central to the discovery of the quark-gluon plasma at the Relativistic Heavy Ion Collider (RHIC)," said Jiangyong Jia, a physicist and professor at Stony Brook University and Brookhaven Lab, where RHIC operates as a DOE Office of Science user facility for nuclear physics research. Jia conducts research at both RHIC and the LHC and led the new ATLAS analysis.

"The new results from ATLAS, while confirming the fluid-like nature of the QGP (quark-gluon plasma), also reveal something new because the type of flow we studied, 'radial' flow, has a different geometric origin from the 'elliptic' flow studied previously, and it is sensitive to a different type of viscosity in the fluid system," Jia said.

The findings offer new insight into the nature of the hot, dense matter generated in these collisions -with temperatures more than 250,000 times hotter than the sun's core. These extreme conditions essentially melt the protons and neutrons that make up the colliding ions, setting free their innermost building blocks, quarks and gluons, to create a quark-gluon plasma.

Read the full story on the Brookhaven National Laboratory website.