  The charged particles produced in nucleus-nucleus collisions come from
leading particles and those frozen out from the hot and dense matter created in
collisions. The leading particles are conventionally supposed having Gaussian
rapidity distributions normalized to the number of participants. The hot and
dense matter is assumed to expand according to the unified hydrodynamics, a
hydro model which unifies the features of Landau and Hwa-Bjorken model, and
freeze out into charged particles from a space-like hypersurface with a proper
time of Tau_FO . The rapidity distribution of this part of charged particles
can be derived out analytically. The combined contribution from both leading
particles and unified hydrodynamics is then compared against the experimental
data performed by BNL-RHIC-PHOBOS Collaboration in different centrality Cu-Cu
collisions at sqrt(s_NN)=200 and 62.4 GeV, respectively. The model predictions
are in well consistent with experimental measurements.
