Azimuthal anisotropies in Pb+Pb and p+Pb collisions with the ATLAS detector

• Flow harmonics measured in the Pb+Pb system agree with the viscous hydrodynamics.
• Similar studies in p+Pb collisions reveal intriguing similarities to Pb+Pb results.
• Results indicate the importance of final-state effects in small collision systems.

An overview of the results on anisotropic particle flow in lead–lead collisions at sNN=2.76TeV and proton–lead collisions at sNN=5.02TeV obtained with the ATLAS detector at the Large Hadron Collider is presented. The final-state anisotropy is studied by measuring the Fourier harmonics (vnvn) of the azimuthal angle distributions of produced charged particles. A large anisotropy in the azimuthal angle distribution of particles produced in Pb+Pb collisions is measured, providing new constraints on the initial geometry models and on hydrodynamic evolution of the system. The same analysis methods have been also applied to study modulations in the azimuthal angle distributions of particles produced in p+Pb collisions. The obtained results are indicative of the importance of final-state effects in small collision systems which may lead to collective flow similar to that observed in the dense and hot system created in Pb+Pb collisions.