Parton Distributions of the Virtual Photon

We propose a generic ansatz for the extension of parton distributions of the real photon to those of the virtual photon. Alternatives and approximations are studied that allow closed-form parametrizations. a Heisenberg Fellow. CERN--TH/96--04 1 Introduction It is well known that the real photon has a partonic substructure, induced by virtual fluctuations into qq pairs. These fluctuations are in part non-perturbative, and so cannot be calculated from first principles (at least not without major advances in lattice-gauge theory) . Only if the parton distributions are specified by hand at some sufficiently large input scale Q 0 can the continued evolution with Q 2 be described perturbatively. Here Q 2 is the scale of the "probing" hard process. Such a partonic substructure also exists for a spacelike photon, P 2 = \Gammap 2 fl ? 0. Only if P 2 is in the deeply inelastic scattering region can the substructure be neglected; the effects here die away like a higher-twist contrib..