Lepton Pair Production at the LHC and the Gluon Density in the Proton

The production of lepton pairs in hadron collisions h1h2 → γX ; γ → ll̄ proceeds through an intermediate virtual photon via qq̄ → γ, and the subsequent leptonic decay of the virtual photon. Interest in this Drell-Yan process is usually focussed on lepton pairs with large mass Q which justifies the application of perturbative QCD and allows for the extraction of the antiquark density in hadrons [ 1]. Prompt photon production h1h2 → γX can be calculated in perturbative QCD if the transverse momentum QT of the photon is sufficiently large. Because the quark-gluon Compton subprocess is dominant, gq → γX , this reaction provides essential information on the gluon density in the proton at large x [ 2]. Alternatively, the gluon density can be constrained from the production of jets with large transverse momentum at hadron colliders [ 3]. In this report we exploit the fact that, along prompt photon production, lepton pair production is dominated by quark-gluon scattering in the region QT > Q/2. This realization means that new independent constraints on the gluon density may be derived from Drell-Yan data in kinematical regimes that are accessible at the Large Hadron Collider (LHC) but without the theoretical and experimental uncertainties present in the prompt photon case. In leading order (LO) QCD, two partonic subprocesses contribute to the production of virtual and real photons with non-zero transverse momentum: qq̄ → γg and qg → γq. The cross section for lepton pair production is related to the cross section for virtual photon production through the leptonic branching ratio of the virtual photon α/(3πQ). The virtual photon cross section reduces to the real photon cross section in the limit Q → 0. The next-to-leading order (NLO) QCD corrections