Low mass lepton pair production at large transverse momentum

PHENIX collaboration has recently measured the transverse momentum distribution of lepton pair production at RHIC with the pair's invariant mass as low as 120 $< Q <$ 300 MeV. We will show that the distribution of low mass lepton pair production at large transverse momentum $Q_T \gg Q$ can be systematically calculated in terms of the perturbative QCD factorization approach. All factorized short-distance parotnic hard parts are evaluated at a distance scale $\sim 1/Q_T$, while all long-distance non-perturbative physics are factorized into the universal parton-to-lepton pair fragmentation functions. We introduce a model for the input lepton pair fragmentation functions at a scale $\mu\sim$ 1 GeV, which are then evolved perturbatively to scales relevant at RHIC. Using the evolved fragmentation functions, we calculate the transverse momentum distributions of low mass lepton pair production in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions. We demonstrate that the transverse momentum distribution of low mass lepton pairs is extremely sensitive to the shape of gluon distribution.