Centrality and momentum-selected elliptic flow: Tighter constraints for the nuclear equation of state

Proton elliptic flow is studied as a function of impact parameter b, for two transverse momentum cuts in 2--6AGeV Au+Au collisions. The elliptic flow shows an essentially linear dependence on b (for 1. 5<b<8fm) with a negative slope at 2AGeV, a positive slope at 6AGeV, and near zero slope at 4AGeV. These selective flow measurements provide better understanding of the interplay of the different factors responsible for the generation of elliptic flow at AGS energies. In addition, extensive comparisons of the measured and calculated flow values indicate that such measurements offer much more stringent constraints for discriminating between various equations of state.