Imprinting Quantum Fluctuations on Hydrodynamic Initial Conditions

J. Scott Moreland, Duke University

Ultra-relativistic heavy-ion collisions allow physicists to recreate and study the extreme conditions that existed mere moments after the big bang when temperatures exceeded 2 trillion degrees Celsius and nuclear matter existed as a hot soup of quarks and gluons known as a quark-gluon plasma (QGP). The success of ideal hydrodynamics in describing QGP phenomena has generated intense interest in its hydrodynamic transport coefficients, specifically, its shear viscosity to entropy ratio η/s.

Efforts to extract the QGP shear viscosity using hydrodynamic simulations are highly sensitive to uncertainties in the theoretical models used to initialize the QGP fireball. Previous studies indicate that accurate reproductions of the centrality-dependent elliptic and triangular flows require initial conditions that incorporate event-by-event fluctuations in the nucleon distributions of colliding nuclei [1]. In addition to these nucleon fluctuations, one should also expect quantum fluctuations in the quark and gluon fields of participant nucleons.

Starting from the two-point covariance function derived in [2], we have developed a Monte Carlo algorithm to generate gluonic fluctuations in the transverse energy density profile predicted by Color-Glass Condensate initial conditions. We find that these quantum fluctuations have little to no effect on the low-order eccentricity coefficients predicted by the MC-KLN model and have only a small effect on higher-order terms. These findings disagree with some of the results reported in [3], and we discuss possible origins for this discrepancy. Our results imply that an earlier extraction of the QGP shear viscosity from a combined analysis of elliptic and triangular flow data from Pb-Pb collisions at the LHC [4] is robust.

[1] B. Alver et al. [PHOBOS Collaboration], “Event-by-Event Fluct. of Azimuthal Particle Anisotropy in Au+Au Collisions at √s = 200 GeV” Phys. Rev. Lett. 98, 242302 (2007).
[2] B. Muller and A. Schafer, “Transverse energy density fluctuations in the Color Glass Condensate Model,” arXiv:1111.3347 [hep-ph].
[3] B. Schenke, P. Tribedy and R. Venugopalan, “Fluctuating Glasma initial conditions and flow in heavy ion collisions,” arXiv:1202.6646 [nucl-th]; F. Gelis, T. Lappi and L. McLerran, “Glittering Glasmas,” Nucl. Phys. A 828, 149 (2009); A. Dumitru and Y. Nara, “KNO scaling of fluctuations in pp and pA, and eccentricities in heavy-ion collisions,” Phys. Rev. C85, 034907 (2012).
[4] Z. Qiu, C. Shen and U. Heinz, “Hydrodynamic elliptic and triangular flow in Pb-Pb collisions at sqrt(s)=2.76ATeV,” Phys. Lett. B707, 151 (2012).

Abstract Author(s): J.S. Moreland, Z. Qiu, U. Heinz