Harmonic decomposition of two particle angular correlations in Pb–Pb collisions at root s NN = 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 root s (NN) = 2.76 TeV for transverse momenta 0.25<Pt<15 GeV/c. 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 Δϕ≡ϕt−ϕa are analyzed for |Δη|≡|ηt−ηa|>0.8, and are referred to as “long-range correlations”. Fourier components VnΔ≡〈cos(nΔϕ)〉 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 is fully described in terms of single-particle anisotropies vn(pT) as . This expectation is tested for 1⩽n⩽5 by applying a global fit of all 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 . For the n=1 harmonic, however, a single v1(pT) curve is not obtained even within the reduced range .