Elliptic flow at forward rapidity in sqrt(s_(NN)) = 200 GeV Au+Au collisions

Abstract

Forward rapidity elliptic flow (v2) of both unidentified charged hadrons and decay muons has been measured from √sNN = 200 GeV Au+Au collisions as a function of pseudorapidity (η), transverse momentum, and number of nucleon collision participants. The measurements were performed at Brookhaven National Laboratory's Relativistic Heavy Ion Collider using the PHENIX experiment's Muon Arm spectrometers, located at 1.2 < |η| 􏰁 2.4. To identify hadrons, which consist mostly of pions, kaons, and protons, a longitudinal momentum cut was applied to tracks stopping in the shallow steel layers of the Muon Arms. Those particles traversing completely through the Muon Arms consist of mostly muons from pion and kaon decays. The standard event plane (EP) method was used to measure v2, whose accuracy was improved ∼20-25% by combining the measured EP angles of several detectors, instead of using the measured EP from a single detector. Additionally, a hit swapping technique was devised to optimize track cuts, estimate background, and apply a background correction. To investigate the ability of the Muon Arms to accurately measure unidentified hadron v2, a GEANT simulation was also undertaken. The forward rapidity v2 results show good agreement with mid-rapidity measurements for central collisions (􏰁 20-30% centrality), indicating a longitudinally extended thermalized medium with similar eccentricity, at least out to the Muon Arm η region. Only when compared to very forward BRAHMS measurements (η ≈ 3) is a v2 suppression seen for central collisions. For increasingly peripheral collisions, a growing suppression in v2 is observed for the Muon Arm measurements compared to mid-rapidity, indicating increased changes in the medium properties of ever smaller systems. For peripheral collisions of the same/similar centralities, an increased suppression is observed toward forward η.