Triggered di-hadron correlations in Pb–Pb collisions from the ALICE experiment

Abstract

Angular correlations between trigger (t) and associated (a) hadrons are measured by the ALICE experiment for 0.5 < pt, aT < 15 GeV, where ptT ⩾ pTa. The azimuthal pair correlation shapes are examined in a variety of centrality categories for pairs in |η| < 1.0 where |ηt − ηa| > 0.8. A series of two-particle Fourier components VnΔ ≡ ⟨cos (nΔϕ)⟩ are extracted from the long-range azimuthal correlation functions. The sum of n < 6 terms matches the data. For each n, a fit is applied over all pT bins simultaneously to test the collectivity hypothesis VnΔ ≃ vtn van. The factorization holds at pt, aT below approximately 4 GeV but breaks progressively at higher momenta. The divergence between the data and the global fit quantifies the onset of nonflow dominance in long-range correlations due to the away side jet. The vn values from the global fit are in close agreement with results from more established methods. At higher pT where jet correlations dominate, the modification of conditional yields in central Pb–Pb collisions is measured with respect to pp (IAA) and with respect to peripheral events (ICP). Significant suppression is observed on the side opposing the trigger, while a moderate enhancement is measured on the near side.