  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 M_t 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 M_t scaling resulting from these
effects are evaluated as a function of the shear viscosity to entropy ratio
(eta/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. Due to different kinematics,
for a given M_t , 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.
