Soft probes of p+Pb and Pb+Pb collisions in the ATLAS experiment at the LHC

Collective flow and azimuthal correlation measurements of particles in the $p_T$ region $<\approx$ 10 GeV provide valuable information in constraining the initial conditions and expansion dynamics of the system produced in heavy-ion collisions. ATLAS collaboration has recently measured correlations between flow harmonics in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, using an event-shape selection procedure, and has separated the components arising from linear and non-linear hydrodynamic evolution in higher order harmonics ($v_4$ and $v_5$). A brief overview of these results are presented in this proceedings. Recently, azimuthal correlations extending to large pseudorapidity differences ('ridge'), similar to that observed in heavy-ion collisions, was observed in p+Pb collisions at LHC. This proceedings also present a measurement of the Fourier harmonics associated with the ridge correlations ($v_1$ to $v_5$) in $p$+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV. The results are compared with $v_n$ in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV with similar event multiplicities. Reasonable agreement is observed after accounting for the difference in the average $p_T$ of particles produced in the two collision systems, consistent with a recent hydrodynamic calculation.