Stony Brook University
Pennsylvania State University
Stony Brook University
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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