Eccentricity and elliptic flow at fixed centrality in Au+Au collisions at sqrt(s_NN)=200GeV in AMPT

To reduce the effect of the fluctuations of initial geometrical shape, elliptic flow are studied at fixed centrality in Au+Au collision at $\sqrt{s_{\rm NN}}$=200GeV. It is observed with the participant increasing, elliptic flow has an increase and a decrease at different fixed impact parameter, but not a trivial fluctuation. It is analyzed that the initial space anisotropy dominates the participant dependence of elliptic flow in near-central collisions(b=5fm) and mid-central collisions(b=8fm), while the interaction between particles can mainly answer for the behavior of elliptic flow in peripheral collisions(b=12fm). To distinguish the pure geometrical effect, elliptic flow scaled by initial eccentricity is studied. It is found that the ratio $v_{2}/\epsilon$ increases with participant and reaches a saturation when the participant is large enough, indicating that the collision system reach the local equilibrium.