Parton model with transverse momentum

The impulse approximation as it is used in lepton-hadron deep-inelastic scattering is studied with unconstrained transverse momentum and with inclusion of binding effects. The use of Sudakov parametrization allows us to write the general form of the quark momentum in a nice way. The maximal extension of the Nachtmann variable is derived and its properties are analyzed. The general kinematics in different frames is briefly considered. The general forms of the elementary cross section and of the structure functions are written down. A phenomenological way to determine the effective masses of the quarks is proposed. A physical picture of the interaction is suggested which is compatible with the old-fashioned naive quark model. A general expression of R = sigma/sub L//sigma/sub T/ is written down. It displays a xi and Q2 dependence which will make very difficult the separation between quantum-chromodynamics (QCD) and binding (or transverse-momentum) effects. The analysis does not make any specific dynamical assumption except for the well understood asymptotic-freedom property of QCD