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

Multi-particle cumulants and correlations of two flow harmonics vn and vm via three- and four-particle symmetric and asymmetric cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four pseudorapidity-separated subevents. The results show an anti-correlation between $v_{2}$ and $v_{3}$ and a positive correlation between $v_{2}$ and $v_{4}$ for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend strongly on the event multiplicity, transverse momentum and the collision systems. 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. Previous measurements based on the standard cumulant method are contaminated by large non-flow background, while the new results based on the subevent methods provide strong evidence for a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions.