Measurement of long-range multi-particle azimuthal correlations with the subevent cumulant method in pp and p+Pb collisions with the ATLAS detector at the LHC

Abstract

A detailed study of multi-particle azimuthal correlations is presented using pp data at √s = 5.02 and 13 TeV, and p+Pb data at √sNN = 5.02 TeV, recorded with the ATLAS detector at the LHC. The azimuthal correlations are probed using four-particle cumulants cn{4} and flow coefficients vn{4} = (−cn{4})1/4 for n = 2 and 3, with the goal of ex- tracting long-range multi-particle azimuthal correlation signals and suppressing the short- range correlations. The values of cn{4} are obtained as a function of the average number of charged particles per event, ⟨Nch⟩, using the recently proposed two-subevent and three- subevent cumulant methods, and compared with results obtained with the standard cumulant method. The standard method is found to be strongly biased by short-range correlations, which originate mostly from jets with a positive contribution to cn{4}. The three-subevent method, on the other hand, is found to be least sensitive to short-range correlations. The three-subevent method gives a negative c2{4}, and therefore a well-defined v2{4}, nearly independent of ⟨Nch⟩, which implies that the long-range multi-particle azimuthal correla- tions persist to events with low multiplicity. Furthermore, v2{4} is found to be smaller than the v2{2} measured using the two-particle correlation method, as expected for long-range collective behavior. Finally, the measured values of v2{4} and v2{2} are used to estimate the number of sources relevant for the initial eccentricity in the collision geometry. The re- sults based on the subevent cumulant technique provide direct evidence, in small collision systems, for a long-range collectivity involving many particles distributed across a broad rapidity interval.