Measurement of long-range azimuthal correlations in proton-proton and proton-lead collisions with ATLAS

Measurement of correlations between two flow harmonics using four-particle symmetric and three-particle asymmetric cumulants are presented with the ATLAS detector in different collision systems - pp, p+Pb, and Pb+Pb to probe the long-range collective nature of particle production in small systems. The problem of non-flow contamination of the standard cumulants in small system is solved using the subevent method technique. Systematic and significant reduction of non-flow using the three-subevent method is shown which is consistent with four-subevent method. Anti-correlation between $v_2$ and $v_3$ and positive correlation between $v_2$ and $v_4$ for all collision systems over the full multiplicity range is observed with three-subevent method. The relative correlation strength, obtained by appropriate normalization of the cumulants with the $\langle v_n^2\rangle$ from a two-particle correlation analysis is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. The results based on the three-subevent methods provide strong evidence for a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions.