Drell-Yan lepton pair azimuthal asymmetry in hadronic processes

We study the azimuthal asymmetry (cos2ϕ) in the Drell-Yan lepton pair production in hadronic scattering processes at moderate transverse momentum region, taking into account the contributions from the twist-three quark-gluon correlations from the unpolarized hadrons. The contributions are found to dominate the asymmetry, and are not power suppressed by q⊥/Q at small q⊥ where q⊥ and Q are the transverse momentum and invariant mass of the lepton pair. Accordingly, the Lam-Tung relation will be violated at this momentum region, and its violation depends on the twist-three functions. However, at large transverse momentum q⊥∼Q, the Lam-Tung relation still holds because all corrections are power suppressed by Λ2/q⊥2∼Λ2/Q2 where Λ is the typical nonperturbative scale.