Highly Sensitive Centrality Dependence of Elliptic Flow: A Novel Signature of the Phase Transition in QCD

Elliptic flow of the hot, dense system which has been created in nucleus-nucleus collisions develops as a response to the initial azimuthal asymmetry of the reaction region. Here it is suggested that the magnitude of this response shows a highly sensitive dependence on the centrality of collisions for which the system passes through a first-order or rapid transition between quark-gluon plasma and hadronic matter. We have studied the system $\mathrm{Pb}\left(158A\mathrm{GeV}\right)$ on Pb employing a recent version of the transport theoretical approach relativistic quantum molecular dynamics and find the conjecture confirmed. The novel phase transition signature may be observable in present and forthcoming experiments at CERN-SPS and at RHIC, the BNL collider.