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Collectivity in Small Colliding Systems with High Multiplicity


The particle angular correlation measurements in small colliding systems, such as p+p, p+Pb, d+Au and 3He+Au at RHIC and the LHC, have recently attracted significant interest. In particular, high-multiplicity events from such collisions exhibit a very similar pattern of long-rapidity-range azimuthal correlations (the "ridge") as in Au+Au and Pb+Pb collisions. In AA collisions it is widely accepted that the ridge reflects various harmonic flow components arising from strong hydrodynamic response to fluctuating initial conditions. Hence, it is natural to ask whether the ridge observed in small colliding systems may also be interpreted as hydrodynamic flow plus fluctuating initial conditions.

Although such a hydro-based interpretation can be made to work phenomenologically, it is unclear whether the hydrodynamic gradient expansion approach developed for A+A collisions is applicable in such small systems. Besides, there are alternative proposals in explaining the observation based on initial state effects (e.g. early gluon dynamics in a saturation picture).

More recently, multi-particle cumulants measurements (as reported at QM14) have shown strong evidence for collectivity in high multiplicity p+Pb collisions, but the exact physical meaning of these measurements needs to be clarified. This Workshop specifically aims to have focused discussions on these issues, and hopefully progress will be made in understanding them.

Specific topics to be addressed include:

  1. Meaning of experimental measurements: What are the precise meanings of cumulants measurements? Whether/how do they unambiguously prove collectivity? What are the effects of potential event centrality selection bias, as well as the roles of jets and other short range correlations?
  2. Meaning and origin of collectivity: What are the possible explanations for observed ridge correlations? How well do they explain the features in the data? What are the possible hydrodynamic as well as non-hydrodynamic origins of collectivity?
  3. Validity of hydrodynamics: Theoretically, what is the boundary for the applicability of current viscous hydrodynamic modeling? Suppose hydrodynamics could indeed work for such small colliding systems, whether/how can we understand it in the usual hydrodynamic gradient expansion framework? What are the roles of hydrodynamic fluctuations which become significant in small systems?


Presentations will be by invitation only.

Evening Event

Registered attendees are invited to attend the optional banquet dinner; free of charge for registered participants.

Banquet Dinner
Berkner Hall (Bldg. 488)
Main Lobby
Located on Brookhaven Avenue
Thursday, March 5, 2015

Workshop Organizers


Click on the "Workshop Information" tab above to find accommodation information.

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Last Modified: December 27, 2017