Radiatively generated parton distributions of real and virtual photons

The parton content of real ${(P}^2=0)$ and virtual ${(P}^2\ne 0)$ transverse photons $\gamma {(P}^2)$ is expressed in terms of perturbative pointlike and nonperturbative hadronic (VMD) components, employing recently updated parton distributions of pions and protons. The resulting parameter-free and perturbatively stable LO and NLO parton densities $f^{\gamma {(P}^2)}{(x,Q}^2)$ are smooth in $P^2$ and apply to all $P^2>~0$ whenever $\gamma {(P}^2)$ is probed at scales $Q^2\gg P^2$ where transverse photons also dominate physically relevant cross sections. Predictions are given for the structure function $F_2^{\gamma {(P}^2)}{(x,Q}^2)$ and $f^{\gamma {(P}^2)}{(x,Q}^2),$ and are compared with all relevant data for real photons as well as with recent data for virtual photons as extracted from DIS $\mathrm{ep}$ dijet events. Simple analytic parametrizations of our predicted parton distributions are presented for the real photon in LO and NLO, and for the virtual photon in LO which, within sufficient accuracy, may be also used in NLO-QCD.