LOW-MASS AND HIGH-MASS COMPONENTS OF THE PHOTON DISTRIBUTION-FUNCTIONS

The structure of the general solution of the inhomogeneous evolution e quations allows the separation of a photon structure,function into per turbative (''anomalous'') and non-perturbative contributions. The form er part is fully calculable, and can be identified with the high-mass contributions to the dispersion integral in the photon mass. Properly normalized ''state'' distributions can be defined, where the gamma --> q $($) over bar$$ q splitting probability is factored out. These stat e distributions are shown to be useful in the description of the hadro nic event properties, and necessary for a proper eikonalization of jet cross sections. Convenient parametrizations are provided both for the state and for the full anomalous parton distributions. The non-pertur bative parts of the parton distribution functions of the photon are id entified with the low-mass contributions to the dispersion integral. T heir normalizations, as well as the value of the scale Q(o) at which t he perturbative parts vanish, are fixed by approximating the low-mass contributions by a discrete, finite sum of vector mesons. The shapes o f these hadronic distributions are fitted to the available data on F-2 (gamma) (x,Q(2)). Parametrizations are provided for Q(o) = 0.6 GeV and Q(o) = 2 GeV, both in the DIS and the MS factorization schemes. The f ull parametrizations are extended towards virtual photons. Finally, th e often-used ''FKP-plus-TPC/2 gamma'' solution for F-2(gamma)(x, Q(2)) is commented upon.