Harmonic decomposition of two particle angular correlations in Pb–Pb collisions at √sNN=2.76 TeV

Article history: Received 22 September 2011 Received in revised form 23 January 2012 Accepted 24 January 2012 Available online 28 January 2012 Editor: V. Metag Angular correlations between unidentified charged trigger (t) and associated (a) particles are measured by the ALICE experiment in Pb–Pb collisions at √ sNN = 2.76 TeV for transverse momenta 0.25 < p T < 15 GeV/c, where p t T > p a T . The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval |η| < 1.0. Distributions in relative azimuth φ ≡ φt − φa are analyzed for | η| ≡ |ηt − ηa| > 0.8, and are referred to as “long-range correlations”. Fourier components Vn ≡ 〈cos(n φ)〉 are extracted from the long-range azimuthal correlation functions. If particle pairs are correlated to one another through their individual correlation to a common symmetry plane, then the pair anisotropy Vn (pT , p a T ) is fully described in terms of single-particle anisotropies vn(pT ) as Vn (pT , p a T ) = vn(pT )vn(pT ). This expectation is tested for 1 n 5 by applying a global fit of all Vn (pT , p a T ) to obtain the best values vn{GF}(pT ). It is found that for 2 n 5, the fit agrees well with data up to pT ∼ 3–4 GeV/c, with a trend of increasing deviation as pT and pT are increased or as collisions become more peripheral. This suggests that no pair correlation harmonic can be described over the full 0.25< pT < 15 GeV/c range using a single vn(pT ) curve; such a description is however approximately possible for 2 n 5 when pT < 4 GeV/c. For the n = 1 harmonic, however, a single v1(pT ) curve is not obtained even within the reduced range pT < 4 GeV/c. © 2012 CERN. Published by Elsevier B.V. All rights reserved.