Two-particle correlations in p-p and Pb-Pb collisions with the ATLAS detector at the LHC

An overview of the angular correlation analyses in ATLAS are presented. Measurements of two-particle angular correlations in proton-proton collisions at centre-of-mass energies of 900 GeV and 7 TeV are presented. Correlations are measured for charged particles in the kinematic region pT > 100 MeV and |η|<2.5 in Minimum Bias events, using data collected by the ATLAS detector at the LHC during 2009 and 2010. Results are compared to different Monte Carlo simulations. It has been realized recently that the flow harmonics beyond second order are important components of the collective phenomena in heavy ion collisions, possibly due to initial geometry fluctuations and hot spots. These flow harmonics are sub-leading in non-central collisions but can have a large magnitude in central collisions where fluctuations dominate the initial nucleon configurations. Such flow harmonics are also important for interpreting the two particle correlation in relative azimuth (Deltaphi) and pseudo-rapidity (Deltaeta).