The Transverse Energy as a Barometer of a Saturated Plasma

The evolution of the gluon plasma produced with saturation initial conditions is calculated via Boltzmann transport theory for nuclear collisions at high energy. The saturation scale increases with the nuclear size and the beam energy, and thus we find that the perturbative rescattering rate decreases relative to the initial longitudinal expansion rate of the plasma. The effective longitudinal pressure remains significantly below the lattice QCD pressure until the plasma cools to near the confinement scale. Therefore, the transverse energy per unit of rapidity and its dependence on beam energy provides a sensitive test of gluon saturation models: the fractional transverse energy loss due to final state interactions is smaller and exhibits a weaker energy dependence than if ideal (nondissipative) hydrodynamics applied throughout the evolution.