Measurement of elliptic and higher-order harmonics at 2.76 TeV Pb-Pb collisions with the ATLAS detector

Anisotropy coefficients v_n are important observables for studying the hot, dense medium created in heavy ion collisions. They not only probe the collective flow of the bulk medium (at pT<3-4 GeV), but also probe the path-length dependent energy loss (at higher pT), both are associated with the asymmetries in the initial geometry. However, auto-correlations not related to initial geometry, commonly referred to as non-flow effects, can contribute to these coefficients, hence need to be systematically suppressed. We present comprehensive measurements of coefficients v_2-v_6 using the event plane method and two-particle correlations method in broad p_T, eta and centrality ranges using the Pb-Pb data from the ATLAS experiment. The phase space regions where the two methods are consistent and where they disagree are explored, and the role of harmonic flow, path-length dependence of jet quenching, and non-flow effects in different part of the phase space are clarified. These detailed measurements provide new insights into the hydrodynamic picture at low pT, the jet energy loss picture at high pT, and the nature of the fluctuations in the initial geometry; they also provide a natural explanation for the "ridge" structures observed in two-particle correlation functions.