Rapidity equilibration and longitudinal expansion at RHIC

The evolution of net-proton rapidity spectra with sqrt(s_NN) in heavy relativistic systems is proposed as an indicator for local equilibration and longitudinal expansion. In a Relativistic Diffusion Model, bell-shaped distributions in central collisions at AGS energies and double-humped nonequilibrium spectra at SPS show pronounced longitudinal collective expansion when compared to the available data. The broad midrapidity valley recently discovered at RHIC in central Au+Au collisions at sqrt(s_NN) = 200 GeV indicates rapid local equilibration which is most likely due to deconfinement, and fast longitudinal expansion of the locally equilibrated subsystem. A prediction is made for Au+Au at sqrt(s_NN)= 62.4 GeV.