Transverse mass scaling of dilepton radiation off a quark-gluon plasma

Abstract

The spectrum of dileptons produced by the quark-gluon plasma in an ultrarelativistic nucleus-nucleus collision depends only, to a good approximation, on the transverse mass Mt of the dilepton. This scaling is exact as long as transverse flow is negligible, and the system is in local thermal equilibrium. We implement a state-of-the-art modelization of kinetic and chemical equilibration in the early stages of the evolution to study the modifications of the spectrum. Violations of Mt scaling resulting from these effects are evaluated as a function of the shear viscosity to entropy ratio (η/s) that controls the equilibration time. We determine the dependence of the spectrum on system size, centrality, rapidity, and collision energy. We show that the quark-gluon plasma produces more dileptons than the Drell-Yan process up to invariant masses of order M∼4 GeV at LHC energies. Due to different kinematics, for a given Mt, the dependence of the dilepton yield on M is opposite for the two processes, so that experiment alone can in principle determine which process dominates.