Physical Review C
We present two-dimensional (2D) two-particle angular correlations measured with the STAR detector on relative pseudorapidity η and ϕ for charged particles from Au-Au collisions at √ˢᴺᴺ = 62 and 200 GeV with transverse momentum pt ≥ 0.15 GeV/c, │η│ ≤ 1, and 2 π 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 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.
Original Publication Citation
Agakishiev, G., Aggarwal, M. M., Ahammed, Z., Alakhverdyants, A. V., Alekseev, I., Alford, J., . . . Collaboration, S. (2012). Anomalous centrality evolution of two-particle angular correlations from Au-Au collisions at √ˢᴺᴺ = 62 and 200 GeV. Physical Review C, 86(6), 064902. doi:10.1103/PhysRevC.86.064902
Agakishiev, G.; Aggarwal, M. M.; Bueltmann, S.; Koralt, I.; Plyku, D.; and STAR Collaboration, "Anomalous Centrality Evolution of Two-Particle Angular Correlations from Au-Au Collisions at √ˢᴺᴺ = 62 and 200 GeV" (2012). Physics Faculty Publications. 154.