  We propose a method to separate \Delta\eta-dependent and
\Delta\eta-independent azimuthal correlations using two- and four-particle
cumulants between pseudo-rapidity (\eta) bins in symmetric heavy-ion
collisions. The \Delta\eta-independent correlation may be dominated by harmonic
flows, a global correlation to the common collision geometry. The
\Delta\eta-dependent correlation can be identified as nonflow, particle
correlations unrelated to the common geometry. Our method exploits the \eta
symmetry of the average harmonic flows and is "data-driven." We use the AMPT
and HIJING event generators to illustrate our method. We discuss the decomposed
\Delta\eta-independent and \Delta\eta-dependent correlations regarding flow and
nonflow in the models.
