Effect of quark off-shellness in DIS and the Drell-Yan process

We study higher twist corrections to the perturbative QCD cross sections for ep deep inelastic scattering and the Drell-Yan lepton pair production in pp collision. The corrections arise due to the initial state interaction of the active quarks with the spectator partons. The effect of this interaction is calculated by dressing the incoming quark lines with phenomenological single parameter Breit-Wigner spectral functions and taking into account full off-shell kinematics. The quark width in the proton is estimated by analyzing the data on Drell-Yan triple differential cross section from the experiment E866 at Fermilab.