Measurements of elliptic flow and higher-order Fourier coefficients with the ATLAS experiment

We report the measurements of collective harmonic flow coefficients $v_n$ for $n=2-6$ in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV with the ATLAS detector. They are extracted via an event plane method based on the ATLAS Forward Calorimeter over a broad range in $p_T$, $eta$ and centrality. The $v_n$ coefficients are found to change weakly within $|eta|<2.5$ and all with the same qualitative trend as a function of $p_T$: all rise up to 3-4 GeV, and then fall towards higher $p_T$. The high statistical precision of the $v_2$ term, allow us to clearly determine its trend out to 20 GeV for a broad centrality classes. The same harmonic flow coefficients are also obtained via a Fourier analysis of the two-particle $Deltaphi-Deltaeta$ correlation with a large $Deltaeta$ gap. A detailed comparison with the event plane method allow us to determine the regions of the two-particle phase space (in $p_T$ and $Deltaeta$ and centrality), in which the collective phenomena play a dominating role. The physics implications of these results are discussed in the context of disentangling the presence of flow harmonics and possible signatures of modified jet fragmentation at low $p_T$.