Two- and multi-particle azimuthal correlations in small collision systems with the ATLAS detector

The recent ATLAS results on two- and multi-particle azimuthal correlations of charged particles are presented for $\sqrt{s}$=~5.02 TeV and 13 TeV $pp$, $\sqrt{s_\mathrm{NN}}$= 5.02 TeV $p$+Pb and $\sqrt{s_\mathrm{NN}}$= 2.76 TeV low-multiplicity Pb+Pb collisions. To remove the "non-flow" contribution from the correlations, that arises predominantly from hard-scattering processes, a template fitting procedure is used in the two-particle correlations (2PC) measurements, while for multi-particle correlations the cumulant method is applied. The correlations are expressed in the form of Fourier harmonics $\mathrm{v}_n (n=2,3,4)$ measuring the global azimuthal anisotropy. The measurements presented hereafter confirm the evidence for collective phenomena in $p$+Pb and low-multiplicity Pb+Pb collisions. For $pp$ collisions the results on four-particle cumulants do not demonstrate a similar collective behaviour.