Factorization model for distributions of quarks in hadrons

We consider distributions of unpolarized (polarized) quarks in unpolarized (polarized) hadrons. Our approach is based on QCD factorization. We begin with a study of the basic factorization for the parton–hadron scattering amplitudes in the forward kinematics and suggest a model for non-perturbative contributions to such amplitudes. This model is based on this simple observation: after emitting an active quark by the initial hadron, the remaining set of quarks and gluons becomes unstable, so a description of this colored state can approximately be done in terms of resonances, which leads to expressions of the Breit–Wigner type. Then we reduce these formulas to obtain explicit expressions for the quark–hadron scattering amplitudes and quark distributions in \(K_T\)- and collinear factorizations.