Organizers:
Jiangyong Jia
Stony Brook University
jjia@bnl.gov

Jean-Yves Ollitrault
Saclay
Jean-Yves.Ollitrault@cea.fr

Gunther Roland
MIT
rolandg@mit.edu

Anna Stasto
Pennsylvania State University
astasto@phys.psu.edu

Derek Teaney
Stony Brook University
derek.teaney@stonybrook.edu

Program Coordinator:
Inge Dolan
inge@uw.edu
(206) 685-4286

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INT Program INT-15-2b

Correlations and Fluctuations in p+A and A+A Collisions

July 6 - July 31, 2015


In the last several years a wealth of precision data and new observables probing the collective correlations in nucleus-nucleus (A+A) collisions at RHIC and the LHC have profoundly changed the study of the Quark Gluon Plasma (QGP). The observed correlations provide an extraordinarily detailed test of the hydrodynamic description of heavy-ion collisions, and promise to strongly constrain the transport properties of the QGP. The new observables which probe rare events, mode mixing, flow fluctuations, and higher harmonic flows, are necessarily complex and must be organized and systematically studied in order to realize the full potential of these measurements.

In addition, the observation of very similar correlation patterns in extremely high multiplicity proton nucleus (p+A) events suggests that hydrodynamics may play an important role in very small QCD systems. However, it is unclear whether the hydrodynamic gradient expansion developed for A+A collisions is applicable in such small systems. The p+A events can be used to study limitations of the hydrodynamic approach, and to parameterize the appropriate initial conditions for A+A. The observed long range correlations in high multiplicity p+A are so similar to the A+A results that is difficult to believe that the source of the observed correlations is different in the two colliding systems. Thus, if the gradient expansion does not converge in p+A, then the applicability of hydrodynamics in A+A must be reexamined.

The correlations observed in high multiplicity p+A collisions have also been analyzed within the framework of the Color Glass Condensate (CGC), where these correlations are built into the initial wave functions of the incoming nuclei. The CGC reproduces many of the qualitative features of the measurements. However, the challenge for the CGC is to reproduce the full range of correlation measurements in p+A such as the higher order harmonics and the multiplicity dependence of the observed angular correlations.

To compare the hydrodynamic theory in A+A to the measured correlations, the initial state fluctuations must be inferred from a reasonable microscopic model. The CGC has been used to model the fluctuating hydrodynamic initial conditions, and such hybrid models are remarkably successful in reproducing the higher harmonic flows observed in A+A collisions. It is currently unclear if the full framework of the perturbative saturation is necessary to reproduce the measured correlations, or if these correlations are simply indicative of subnucleonic fluctuations which can be constrained by a few moments measured in p+p and p+A collisions.

The program will focus primarily on understanding the correlations measurements, and improving the theoretical descriptions that make direct contact with these measurements. Approximately one third of the talks should be experimental, and the theoretical talks should directly address the measured data.