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.Comment: 7 pages, 7 figures, revtex 4; The discussion on the non-flow effects
is extended, a new figure (Fig.3) on v2-eccentricity correlation is added,
accepted for publication in Phys. Rev.