RHIC's perfect liquid a study in perfection

June 18, 2013 by Karen Mcnulty Walsh
Bjoern Schenke, right, chats with fellow Brookhaven Lab nuclear theorist Raju Venugopalan, at last summer's Quark Matter meeting in Washington, D.C. Schenke recently won a Young Scientist Prize in nuclear physics from the International Union of Pure and Applied Physics.

(Phys.org) —When heavy ions (the nuclei of heavy atoms such as gold and lead) collide at high energies at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) and Europe's Large Hadron Collider (LHC), the components of the nuclei (protons and neutrons) melt to form a hot soup of their constituent particles, quarks and gluons. A new model that accurately describes the experimentally observed patterns of particles flowing out from this "quark-gluon plasma" (QGP) suggests that the effective shear viscosity, or resistance to flow, is close to the ideal limit used to define a "perfect" fluid.

"Our result is consistent across finer and finer detailed analyses of particle ," said Bjoern Schenke, a Goldhaber Fellow in the nuclear theory group at Brookhaven Lab and a coauthor on a paper describing the analyses in Physical Review Letters published earlier this year.

"These findings help answer the question of how 'perfect' the perfect liquid QGP created at RHIC is—that is, how close the comes to a limit derived from —and how this property varies with temperature. Our findings indicate that viscosity increases away from the ideal limit with the increasing temperatures reached at LHC," Schenke said.

The findings will also help scientists better understand how the internal characteristics of the heavy ions before they collide—particularly dense concentrations of gluons known as color glass condensate—shape the initial collision geometry and rapidly turn into the liquid .

Schenke and his collaborators arrived at these conclusions by systematically studying the directional-dependence of particles flowing out of collisions of heavy ions at both RHIC and the LHC and applying a model and other methods to identify the viscosity that best fit the experimental data at finer and finer levels of detail.

"Particle flow patterns are strongly linked to the initial 'geometry' of the colliding ions," Schenke explained. "This includes the large-scale elliptic shape of the overlap region of two spherical ions colliding not quite head on, as well as more subtle fluctuations of color glass condensate on scales smaller than the individual protons and neutrons that make up each ion."

This video is not supported by your browser at this time.

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.

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 these initial geometries. The lower the viscosity (actually the ratio of shear viscosity to entropy density), the greater the retention of these initial-state characteristics.

The detailed modeling of fluctuations of initial shapes combined with viscous fluid dynamic calculations revealed that the constant viscosity value needed to describe the data at RHIC was almost half the value of that needed to describe the LHC data.

"This suggests that heavy-ion collisions at RHIC produce a perfect fluid that is initially closer to the ideal hydrodynamic limit, a theoretical scenario in which the viscosity is zero," Schenke said.

This result is in line with theoretical expectations that the ratio of shear viscosity to entropy density increases with increasing temperature, because LHC's collisions occur at higher energy and create higher initial temperatures than RHIC's.

"Higher viscosity dampens the flow of particles at large scales, and even more so at finer scales, resulting in less retention of the initial shape in particles flowing from collisions at LHC," Schenke said.

Understanding details of the temperature dependence of the viscosity will help scientists map the "transport properties" of the quark-gluon plasma created in these collisions, including how strongly interacting the particles are and how "perfect" the liquid is, and possibly explore the point at which a gas made of protons and neutrons melts to form QGP, expected to occur when the viscosity is at its lowest level.

Explore further: 'Perfect' Liquid Hot Enough to be Quark Soup (w/ Video)

More information: www.bnl.gov/physics/NTG/people/schenke.php

Related Stories

'Perfect' Liquid Hot Enough to be Quark Soup (w/ Video)

February 15, 2010

Recent analyses from the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference "atom smasher" at the U.S. DOE's Brookhaven National Laboratory, establish that collisions of gold ions traveling at nearly the speed ...

The perfect liquid -- now even more perfect

January 17, 2012

Ultra hot quark-gluon-plasma, generated by heavy-ion collisions in particle accelerators, is supposed to be the "most perfect fluid" in the world. Previous theories imposed a limit on how "liquid" fluids can be. Recent results ...

Quark matter's connection with the Higgs

August 27, 2012

(Phys.org)—You may think you've heard everything you need to know about the origin of mass. After all, scientists colliding protons at the Large Hadron Collider (LHC) in Europe recently presented stunning evidence strongly ...

World's smallest droplet

May 17, 2013

(Phys.org) —Physicists may have created the smallest drops of liquid ever made in the lab. That possibility has been raised by the results of a recent experiment conducted by Vanderbilt physicist Julia Velkovska and her ...

Recommended for you

CERN collides heavy nuclei at new record high energy

November 25, 2015

The world's most powerful accelerator, the 27 km long Large Hadron Collider (LHC) operating at CERN in Geneva established collisions between lead nuclei, this morning, at the highest energies ever. The LHC has been colliding ...

'Material universe' yields surprising new particle

November 25, 2015

An international team of researchers has predicted the existence of a new type of particle called the type-II Weyl fermion in metallic materials. When subjected to a magnetic field, the materials containing the particle act ...

Exploring the physics of a chocolate fountain

November 24, 2015

A mathematics student has worked out the secrets of how chocolate behaves in a chocolate fountain, answering the age-old question of why the falling 'curtain' of chocolate surprisingly pulls inwards rather than going straight ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.