Revealing long-range multiparticle collectivity in small collision systems via subevent cumulants

Multiparticle azimuthal cumulants, often used to study collective flow in high-energy heavy-ion collisions, have recently been applied in small collision systems such as pp and p+A to extract the second-order azimuthal harmonic flow v2. Recent observation of four-, six-, and eight-particle cumulants with "correct sign" c2{4}<0, c2{6}>0, c2{8}<0 and approximate equality of the inferred single-particle harmonic flow, v2{4}≈v2{6}≈v2{8}, have been used as strong evidence for a collective emission of all the soft particles produced in the collisions. We show that these relations in principle could be violated due to the non-Gaussianity in the event-by-event fluctuation of flow and/or nonflow. Furthermore, we show, using pp events generated with the pythia model, that c2{2k} obtained with the standard cumulant method are dominated by nonflow from dijets. An alternative cumulant method based on two or more η-separated subevents is proposed to suppress the dijet contribution. The new method is shown to be able to recover a flow signal as low as 4% imposed on the pythia events, independently of how the event activity class is defined. Therefore the subevent cumulant method offers a more robust way of studying collectivity based on the existence of long-range azimuthal correlations between multiple distinct η ranges. The prospect of using the subevent cumulants to study collective flow in A+A collisions, in particular its longitudinal dynamics, is discussed.