System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at √sNN=200 GeV

We present results on two-particle angular correlations in Cu+Cu and Au+Au collisions at a center-of-mass energy per nucleon pair of 200 GeV over a broad range of pseudorapidity (η) and azimuthal angle (ϕ) values as a function of collision centrality. The PHOBOS detector at the Relativistic Heavy Ion Collider has a uniquely large angular coverage for inclusive charged particles, which allows for the study of correlations on both long- and short-range scales. A complex two-dimensional correlation structure in Δη and Δϕ emerges, which is interpreted in the context of a cluster model. The effective cluster size and decay width are extracted from the two-particle pseudorapidity correlation functions. The effective cluster size found in semicentral Cu+Cu and Au+Au collisions is comparable to that found in proton-proton collisions but a nontrivial decrease in size with increasing centrality is observed. Moreover, a comparison of results from Cu+Cu versus Au+Au collisions shows an interesting scaling of the effective cluster size with the measured fraction of total cross section (which is related to the ratio of the impact parameter to the nuclear radius, b/2R), suggesting a geometric origin. Further analysis for pairs from restricted azimuthal regions shows that the effective cluster size at Δϕ~180° drops more rapidly toward central collisions than the size at Δϕ~0°. The effect of limited η acceptance on the cluster parameters is also addressed, and a correction is applied to present cluster parameters for full η coverage, leading to much larger effective cluster sizes and widths than previously noted in the literature. These results should provide insight into the hot and dense medium created in heavy ion collisions.United States. Dept. of Energy (Grant Nos. DE-AC02-98CH10886, DE-FG02-93ER40802, DE-FG02-94ER40818, DE-FG02-94ER40865, DE-FG02-99ER41099, and DE-AC02-06CH11357)National Science Foundation (Grant Nos. 9603486, 0072204, and 0245011)Polish MNiSW (Grant No. N202 282234 (2008–2010))NSC of Taiwan (Contract No. NSC 89-2112-M-008-024)Hungarian OTKA (Grant No. F 049823