Physically Motivated Approximation to a Parton Distribution Function in QCD

It has been suggested that parton distributions in coordinate space, so called Ioffe-time distributions, provide a more natural object for non-perturbative methods compared to the usual momentum distributions. In this paper we argue that the shape of experimentally determined Ioffe-time distributions of quarks in a nucleon target clearly indicates separation of longitudinal scales, which is not easily recognizable in terms of conventional longitudinal momentum space considerations. We demonstrate how to use this observation to determine parton distributions, using non-perturbative information about the first few moments and the Regge asymptotics at small $x$.