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

Abstract

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 < pt,aT < 15 GeV/c, where ptT > paT. 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(nφ) 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(ptT , paT ) is fully described in terms of single-particle anisotropies vn(pT ) as Vn(ptT , paT ) = vn(ptT )vn(paT ). This expectation is tested for 1 ≤ n ≤ 5 by applying a global fit of all Vn(ptT , paT ) to obtain the best values vn{GF}(pT ). It is found that for 2 ≤ n ≤ 5, the fit agrees well with data up to paT ∼ 3–4 GeV/c, with a trend of increasing deviation as ptT and paT 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 paT < 4 GeV/c. For the n = 1 harmonic, however, a single v1(pT ) curve is not obtained even within the reduced range paT < 4 GeV/c.