Drell-Yan Lepton Pair Azimuthal Asymmetry in Hadronic Processes

We study the azimuthal asymmetry 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_\perp/Q at small q_\perp where q_\perp 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_\perp\sim Q, the Lam-Tung relation still holds because all corrections are power suppressed by \Lambda^2/q_\perp^2\sim \Lambda^2/Q^2 where \Lambda is the typical nonperturbative scale.