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

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

ALICE; LHC; CERN; PbPb; 2.76 TeV; Angular correlations

ALICE Collaboration