Anomalous centrality evolution of two-particle angular correlations from Au-Au collisions atsNN=62and 200 GeV

Abstract

We present two-dimensional (2D) two-particle angular correlations measured with the STAR detector on relative pseudorapidity η and azimuth φ for charged particles from Au-Au collisions at √ s NN = 62 and 200 GeV with transverse momentum p t 0.15 GeV/c, |η| 1, and 2π in azimuth. Observed correlations include a same-side (relative azimuth <π/2) 2D peak, a closely related away-side azimuth dipole, and an azimuth quadrupole conventionally associated with elliptic flow. The same-side 2D peak and away-side dipole are explained by semihard parton scattering and fragmentation (minijets) in proton-proton and peripheral nucleus-nucleus collisions. Those structures follow N -N binary-collision scaling in Au-Au collisions until midcentrality, where a transition to a qualitatively different centrality trend occurs within one 10% centrality bin. Above the transition point the number of same-side and away-side correlated pairs increases rapidly relative to binary-collision scaling, the η width of the same-side 2D peak also increases rapidly (η elongation), and the φ width actually decreases significantly. Those centrality trends are in marked contrast with conventional expectations for jet quenching in a dense medium. The observed centrality trends are compared to perturbative QCD predictions computed in HIJING, which serve as a theoretical baseline, and to the expected trends for semihard parton scattering and fragmentation in a thermalized opaque medium predicted by theoretical calculations and 064902-2 ANOMALOUS CENTRALITY EVOLUTION OF TWO-. . .PHYSICAL REVIEW C 86, 064902 (2012) phenomenological models. We are unable to reconcile a semihard parton scattering and fragmentation origin for the observed correlation structure and centrality trends with heavy-ion collision scenarios that invoke rapid parton thermalization. If the collision system turns out to be effectively opaque to few-GeV partons the present observations would be inconsistent with the minijet picture discussed here.

Third harmonic flow of charged particles in Au+Au collisions atsNN=200 GeV

“…The rise and then fall of the width of v 3 {2}( η) mimics the rise and fall of the low p T ridge amplitude reported in Ref. [30]. Reference [7] describes this centrality trend in terms of participant eccentricity fluctuations, where the fluctuations in midcentral collisions are well above statistical expectations.…”

Mean transverse momenta correlations in hadron-hadron collisions in MC toy model with repulsing strings

“…The motivation for development of such a model comes, for instance, from the results of dihadron correlations measured in Au-Au collisions in STAR [7], where patterns of the collective behavior are observed and a detailed fit of the correlation structures was developed. It is interesting to see what would be the "collectivity" in the frame of the toy MC model with repulsion strings.…”

Correlations and Event-by-Event Fluctuations in High Multiplicity Events Produced in 208Pb-208Pb Collisions

“…Besides this, a significant contribution to the two-particle correlations comes from the collective effects. They appear as a modulation in Δ and are usually searched for in high multiplicity events [46,48]. An attempt is, therefore, made to study the two-particle correlations in individual events exhibiting large fluctuations in their and distributions which have been sorted out as discussed in the previous sections.…”