Elliptic Flow Analysis at RHIC: Fluctuations vs. Non-Flow Effects

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The cumulant method is applied to study elliptic flow (v₂) in Au+Au collisions at √s=200AGeV, with the UrQMD model. In this approach, the true event plane is known and both the non-flow effects and event-by-event spatial (ε) and v₂ fluctuations exist. Qualitatively, the hierarchy of v₂'s from two, four and six-particle cumulants is consistent with the STAR data, however, the magnitude of v₂ in the UrQMD model is only 60% of the data. We find that the four and six-particle cumulants are good measures of the real elliptic flow over a wide range of centralities except for the most central and very peripheral events. There the cumulant method is affected by the v₂ fluctuations. In mid-central collisions, the four and six-particle cumulants are shown to give a good estimation of the true differential v₂, especially at large transverse momentum, where the two-particle cumulant method is heavily affected by the non-flow effects.