Y. Hama
University of São Paulo

J. Jalilian-Marian
Baruch College, CUNY

L. McLerran
Brookhaven National Laboratory

T. Trainor
University of Washington

F. Wang
Purdue University

C-Y. Wong
Oak Ridge National Laboratory

Program Coordinator:
Inge Dolan
(206) 685-4286


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(Deadline January 15, 2012)

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INT Workshop INT-12-51W

The "Ridge" Correlation in High-Energy Collisions at RHIC and LHC

May 7 - 11, 2012

Measurements of two-particle correlations in high-energy nuclear collisions at the RHIC and LHC have provided a wealth of information that can be utilized to deduce the dynamics of the collision process and the properties of the produced dense hot medium. One of the most striking features is a "ridge" structure in Δϕ - Δη angular correlations, where Δϕ and Δη are the azimuthal angle and pseudorapidity differences of two produced hadrons, respectively. Ridge structures along Δη appear on the same side (near side, Δϕ ∼ 0) and on the away side (Δϕπ), with or without a high pT trigger. The "ridge" structure appearing on the same side is relatively narrow in Δϕ but quite extended in Δη. Same-side Ridge structures have been observed in high energy Au+Au and Cu+Cu collisions at the RHIC, and in p+p and Pb+Pb collisions at the LHC.

The general occurrence of ridge correlations for a variety of collision conditions and in several different phase- space regions has stimulated many theoretical discussions. The existence of long-range correlations between produced particles with large pseudorapidity differences poses an interesting theoretical puzzle. It appears timely to gather together physicists working on this interesting topic to examine what the experimental measurements reveal and what the theoretical descriptions entail. For such a purpose, the Ridge Workshop will be organized according to the following structure:

(A) The Inclusive Ridge (or minimum-pt-bias ridge, with no high pt selection, and no vn subtraction):

  • In near side, abrupt rise of ridge yield as a function of centrality.

  • In away side, yield in excess of pQCD correlation predictions for high-multiplcity central collisions.

  • (pT1, pT2) correlations and their centrality dependence.

  • Connections of fit model elements to physics mechanisms.

  • The above interesting ridge properties have been observed at RHIC, what about LHC?

  • Why do the pT of the associated ridge particles peak at a particular pT range in pp collisions (CMS)?

  • If v2 background is somehow subtracted, how are the correlation structures affected?

(B) The High-pT Ridge (with a high-pT trigger, and v2 subtraction):

  • In near side, ridge yield gradually rises in variation with centrality.

  • Three-particle correlations, in-plane and out-of-plane two-particle correlations.

  • What is the amplitude of the ridge as a function of trigger pT or associated particle pT?

  • What happens for RHIC data if v2, v3, v4,... background are subtracted? Would there be a ridge at all after the subtraction?

  • What do these correlations tell us? To what extend do the experimental vn quantities explain all of the above experimental two-body correlation features (including the ridge)?

  • What are the properties of particles in the ridge: pT, flavor composition, etc.?

  • To what degree are the different ridges the same between RHIC and LHC?

    We need data here from both RHIC and LHC.

(C) Theoretical Descriptions

  • To what extent do mini-jets contribute to the global vn, two-body correlations, with and without a high pT trigger?

  • Different theoretical models of the ridge: hydrodynamic flows, local hot spots, initial-state fluctuations, parton cascades, glasma flux tubes, glasma turbulence fields, the momentum kick model, pQCD modeling, etc..

  • How does the inclusive (or minimum-pt-bias) ridge related to the high pT ridge?

  • vn for large n and analogies with the cosmic background radiation.

  • Is vn the same when measured at different rapidities in the same event?

  • To what degree do theoretical considerations depend upon the transverse graininess of the initial conditions? Is there an intrinsic and quantum limitation of theoretical consideration if the initial conditions are too grainy?

  • More generally, what are the intrinsic limitations and model assumptions of various computations of ridge properties?

    We need data here from both RHIC and LHC.

(D) The pp Ridge

  • What is the current experimental situation?

  • To what degree can the ridge be associated with intrinsic correlation in the emission process? With final state interactions or nascent flow?

  • What is the relation between the pp ridge at LHC and the AA ridge at RHIC?

(E) The Intrinsic Strength of Long-Range Rapidity Correlations:

  • How does the long range correlation depend upon centrality, beam energy etc.?

  • In pp or LHC AA Measurements? How to think about it?

  • Multiplicity Fluctuations.

  • If the ridge is associated with an underlying flux tube structure, then there are computable implications concerning the multiplicity fluctuations. Does this work for LHC and RHIC data?

(F) Summaries:

  • Theory: What have been ruled out and what are possibilities. How do we refine our knowledge?

  • Experiment: What needs to be done in future experiments to resolve the underlying physics.

There is a registration fee of $70 to attend this workshop, due in cash or by check drawn on a U.S. bank. Sorry -- we cannot accept credit cards.