Low-mass lepton pair production at large transverse momentum

Abstract

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, ${Q}_{T}\ensuremath{\gg}Q$, with the pair's invariant mass $Q$ as low as $Q\ensuremath{\sim}{\ensuremath{\Lambda}}_{\mathrm{QCD}}$, 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 $\ensuremath{\sim}\mathcal{O}(1/{Q}_{T})$. We introduce a model for the input lepton pair fragmentation functions at a scale ${\ensuremath{\mu}}_{0}\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$, which are then evolved perturbatively to scales relevant at the Relativistic Heavy Ion Collider. Using the evolved fragmentation functions, we calculate the transverse momentum distributions in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions at the Relativistic Heavy Ion Collider. We also discuss the sensitivity of the transverse momentum distribution of low-mass lepton pairs to the gluon distribution.