Drell-Yan lepton angular distribution at small transverse momentum

We investigate the dependence of the Drell-Yan cross section on lepton polar and azimuthal angles, as generated by the lowest-order QCD annihilation and Compton processes. We focus, in particular, on the azimuthal-angular distributions, which are of the form cos and cos2. At small transverse momentum qT of the lepton pair, qTQ, with Q the pair mass, these terms are known to be suppressed relative to the -independent part of the Drell-Yan cross section by one or two powers of the transverse momentum. Nonetheless, as we show, like the -independent part they are subject to large logarithmic corrections, whose precise form, however, depends on the reference frame chosen. These logarithmic contributions ultimately require resummation to all orders in the strong coupling. We discuss the potential effects of resummation on the various angular terms in the cross section and on the Lam-Tung relation. © 2006 The American Physical Society.