Photonic parton distributions

The quark and gluon distributions of the photon are determined in leading and higher order by imposing a vector-meson dominance (VMD) valencelike structure at a low resolution scale adopted from the pion. This leaves only one free parameter, not sufficiently constrained by VMD, to be fixed by experiment. Our predictions are in agreement with presently available data for $F_2^{\gamma }(x,Q^2)$. Simple parametrizations of the resulting parton distributions are presented in the range ${10}^{-5\mathrm{}}\lesssim x<\mathrm{}1\mathrm{}$, $0.3\lesssim Q^2\lesssim {10}^6$ ${\mathrm{GeV}}^2$ as obtained from the leading- and higher-order evolution equations.