Measurement of the azimuthal anisotropy for charged particle production in Pb+Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV and in $p$+Pb collisions at $\sqrt{s_{\rm NN}}$=5.02 TeV with the ATLAS detector at the LHC

In this work, detailed measurements of the $p_{\rm T}$, $\eta$ and centrality dependence of the flow harmonics $v_{n}$ in Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV with the ATLAS detector at the LHC are presented. Consistent values of harmonics $v_{2}$-$v_{6}$ are measured the via event-plane and two-particle correlation methods. Dipolar flow $v_{1}$ associated with initial dipole asymmetry is measured via two-particle correlations. The long-range correlations in the two-particle correlations such as $ridge$ and the $cone$ are shown to be accounted for by collective flow. Sizable values of the odd harmonics especially $v_{3}$ are measured that indicate significant event-by-event (EbyE) fluctuations in the initial geometry. The nature of the initial collision geometry and the hydrodynamic response to it are investigated by measuring the EbyE distributions for harmonics $v_{2}$-$v_{4}$ , as well as correlations between two and three event-planes $\Phi_{n}$ of different orders. These measurements show the presense of considerable EbyE fluctuations in the initial geometry as well as the importance of non-linear hydrodynamic response of the medium to these fluctuations. By measuring the EbyE $v_n$ distributions separately for particles with $p_{\rm T}>1$ GeV and with $p_{\rm T}\in(0.5,1.0)$ GeV, the hydrodynamic response is shown to factorize into a function of $p_{\rm T}$ and a function of the initial geometry. The EbyE $v_n$ distributions are shown to contain additional information than the multi-particle cumulants. Two-particle correlations in $p$+Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV are measured. Long range correlations are observed along $\Delta\eta$ at $\Delta\phi\sim0$ that increase with increasing event-activity. A symmetric long range correlation is also shown to exist at $\Delta\phi\sim\pi$ which is obtained by subtracting out the expected contribution from recoiling dijets. These correlations are shown to be similar to those seen in Pb+Pb collisions suggesting similar physics in both systems.