Revealing long-range multiparticle collectivity in small collision systems via subevent cumulants Article Swipe
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· 2017
· Open Access
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· DOI: https://doi.org/10.1103/physrevc.96.034906
· OA: W2578377470
Multi-particle azimuthal cumulants, often used to study collective flow in\nhigh-energy heavy-ion collisions, have recently been applied in small collision\nsystems such as $pp$ and $p$+A to extract the second-order azimuthal harmonic\nflow $v_2$. Recent observation of four-, six- and eight-particle cumulants with\n"correct sign" $c_2\\{4\\}<0, c_2\\{6\\}>0, c_2\\{8\\}<0$ and approximate equality of\nthe inferred single-particle harmonic flow, $v_2\\{4\\}\\approx v_2\\{6\\}\\approx\nv_2\\{8\\}$, have been used as strong evidence for a collective emission of all\nsoft particles produced in the collisions. We show that these relations in\nprinciple could be violated due to the non-Gaussianity in the event-by-event\nfluctuation of flow and/or non-flow. Furthermore, we show, using $pp$ events\ngenerated with the PYTHIA model, that $c_2\\{2k\\}$ obtained with standard\ncumulant method are dominated by non-flow from dijets. An alternative cumulant\nmethod based on two or more $\\eta$-separated subevents is proposed to suppress\nthe dijet contribution. The new method is shown to be able to recover a flow\nsignal as low as 4\\% imposed on the PYTHIA events, independently of how the\nevent activity class is defined. Therefore the subevent cumulant method offers\na more robust way of studying collectivity based on the existence of long-range\nazimuthal correlations between multiple distinct $\\eta$ ranges. The prospect of\nusing the subevent cumulants to study collective flow in A+A collisions, in\nparticular its longitudinal dynamics, is discussed.\n
