  The production of dileptons with an invariant mass in the range 1 GeV < M < 5
GeV provides unique insight into the approach to thermal equilibrium in
ultrarelativistic nucleus-nucleus collisions. In this mass range, they are
produced through the annihilation of quark-antiquark pairs in the early stages
of the collision. They are sensitive to the anisotropy of the quark momentum
distribution, and also to the quark abundance, which is expected to be
underpopulated relative to thermal equilibrium. We take into account both
effects based on recent theoretical developments in QCD kinetic theory, and
study how the dilepton mass spectrum depends on the shear viscosity to entropy
ratio that controls the equilibration time. We evaluate the background from the
Drell-Yan process and argue that future detector developments can suppress the
additional background from semileptonic decays of heavy flavors.
