Measurement of multi-particle azimuthal correlations in pp, p+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detector

Multi-particle correlations in azimuthal angles of produced charged particles are measured by ATLAS in $\sqrt{s}$ = 5.02 and 13 TeV $pp$ collisions, in $\sqrt{s_{NN}}$ = 5.02 TeV $p$+Pb collisions, and in $\sqrt{s_{NN}}$=2.76 TeV low-multiplicity Pb+Pb collisions at the Large Hadron Collider. The correlations are expressed in terms of cumulants $c_n$, directly related to the $n^{th}$-order Fourier harmonics $\mathrm{v}_n$, measuring the global azimuthal anisotropy of particle production. These measurements aim to assess collective nature of particle production. %While collectivity is well established in collisions involving heavy nuclei, its evidence in $pp$ collisions is less clear. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in $p$+Pb and low-multiplicity Pb+Pb collisions. The same interpretation holds for \pp collisions on the basis of the second-order two-particle cumulants when determined with the requirement of pseudorapidity separation, $|\Delta\eta|>2$, between particles for which the two particle cumulant is evaluated in order to suppress non-flow correlations. On the other hand, the \pp results on four-particle cumulants do not demonstrate the collective behaviour, indicating that they may be biased by contributions from non-flow correlations, i.e. correlations not related to the initial collision geometry. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged particle multiplicity. For a given multiplicity, the strongest collective effects are seen in Pb+Pb, weaker in $p$+Pb and the weakest in $pp$ collisions. The $pp$ results show no dependence on the energy, nor on the multiplicity.