Recent ATLAS results on flow measurements in lead-lead and proton-lead collisions

A detailed measurement of the ATLAS experiment of azimuthal anizotropy in lead-lead and p-lead collisions at the LHC will be presented. Recently, the Fourier coefficients, v2-v4, of the azimuthal angle distribution of charged particles measured in the ATLAS inner detector (|eta| < 2.5) were extracted on event-by-event basis in each of 48 milion minimum-bias Pb+Pb collisions. In particular, the measurements of the vn distributions, unfolded for experimental resolution will be presented. The relationship between the shapes of the vn distributions and the collision geometry and initial-state fluctuations will be discussed. The results will be compared with theoretical calcualtions of initial-state eccentricity distributions. We will also report on a measurement of the anisotropy of charged particle flow in lead-lead collisions using multi-particle cumulant method. The transverse momentum and pseudorapidity dependence of the elliptic flow obtained from the two- and four-particle cumulants will be shown. Event-by-event fluctuations of the elliptic flow have been also evaluated with multi-particle cumulants. Also measurements of the two-particle correlations in relative azimuthal angle (Delta-phi) and pseudorapidity (Delta-eta) in sqrt(s_NN) = 5.02 TeV p+Pb collisions will be presented. The measurements are performed using approximately 1 mub^-1 of data as a function of p_T and the transverse energy (SumE_T) summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Delta-eta|<5) near-side (Delta-phi ~ 0) correlation that grows rapidly with increasing SumE_T. A long-range recoil corrected away-side (Delta-phi ~ pi) correlation, is found to match the near-side correlation in magnitude, shape (in Delta-eta and Delta-phi) and SumE_T dependence. The resultant Delta-phi correlation is approximately symmetric about pi/2, and is consistent with a cos(2Delta-phi) modulation for all SumE_T ranges and particle p_T. The amplitude of this modulation is comparable in magnitude and p_T dependence to similar modulations observed in heavy-ion collisions, suggestive of final-state collective effects in high multiplicity events. In order to study further the long-range correlations ("ridge") observed in p+Pb collisions, the second-order azimuthal anisotropy parameter of charged particles, v2, has been measured with the cumulant method in the same data sample. The parameter v2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |eta|<2.5. The results will be presented as a function of transverse momentum and SumE_T. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions.