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

Abstract: Angular correlations between unidentified hadron trigger and associated particles are measured by the ALICE experiment for $0.5 < p_T^{t,a} < 15$ GeV, where $\ptt \geq \pta$. The pair correlation shapes are examined in a variety of centrality categories for pairs in $|\eta| < 1.0$ where $|\eta^t - \eta^a| > 0.8$. A series of two-particle Fourier components $V_{n\Delta} \equiv <\cos(n\Delta\phi)>$ are extracted from the long-range azimuthal correlation functions. The sum of $n<6$ terms match the data. For each $n$, a fit is applied over all $p_{T}$ bins simultaneously to test the collectivity hypothesis $V_{n\Delta} \simeq v_n^t \, v_n^a$. The factorization holds at $p_{T}^{t,a}$ 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 $v_n$ 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 $pp$ ($I_{AA}$) and with respect to peripheral events ($I_{CP}$). Significant suppression is observed on the side opposing the trigger, while a moderate enhancement is measured on the near side.