How perfect is "perfect"? The Quark-Gluon Plasma at RHIC and LHC

 

Ulrich Heinz

 

The Ohio State University

 

The Quark-Gluon Plasma QGP created in heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) has been called a "perfect liquid".  It exhibits the collective dynamic behavior of a fluid with almost vanishing viscosity.  Theory places lower limits on the shear and bulk viscosities of any real (i.e. not super-) fluid, and the QGP shear viscosity appears to be close to that limit.  Recent progress on the modeling side of heavy-ion collision dynamics has put the quantitative determination of the QGP shear viscosity from experimentally measured hadron spectra and anisotropic flow coefficients within reach.  I will present new limits on the kinematic shear viscosity of QGP extracted from VISHNU, a hydrodynamic code that couples a viscous fluid description of the dense QGP phase with a microscopic Boltzmann cascade for the dilute late hadron resonance gas stage, in comparison with elliptic and triangular flow data from RHIC and LHC.  As will be discussed, event-by-event fluctuations of the initial shape and density distribution in the QGP fireball and the resulting anisotropic flow play a crucial role in this extraction.