  We adopt a factorized QCD formalism to describe the transverse momentum
distribution of low-mass lepton pairs produced in $pp$ collisions, when the
pair transverse momentum $Q_T \gg Q$, with the pair's invariant mass $Q$ as low
as $Q \sim \Lambda_{\mathrm{QCD}}$. We extend this formalism to dilepton
production in $AA$ collisions by including the nuclear-dependent power
correction due to parton multiple scattering.
