Low-mass lepton pair production at large transverse momentum

We study the transverse momentum distribution of low-mass lepton pairs produced in hadronic scattering, using the perturbative QCD factorization approach. We argue that the distribution at large transverse momentum, $Q_T \gg Q$, with the pair's invariant mass $Q$ as low as $Q \sim \Lambda_{\mathrm{QCD}}$, can be systematically factorized into universal parton-to-lepton pair fragmentation functions, parton distributions, and perturbatively calculable partonic hard parts evaluated at a short distance scale $\sim {\cal O}(1/Q_T)$. We introduce a model for the input lepton pair fragmentation functions at a scale $\mu_0\sim 1$ GeV, which are then evolved perturbatively to scales relevant at RHIC. Using the evolved fragmentation functions, we calculate the transverse momentum distributions in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions at RHIC. We also discuss the sensitivity of the transverse momentum distribution of low-mass lepton pairs to the gluon distribution.