The hadronic structure of the photon with emphasis on its two-pion constituent

The hadronic structure of photons is studied by treating the electromagnetic interaction in lowest order. General consequences of this picture and its connection with generalized vector meson dominance for diffractive processes are discussed. Emphasis is given to the dipion constituent which can be regarded approximately as a superposition of two parts: a ${\rho }^0$-meson core and a loose nonresonant two-pion structure. This modification of VMD increases the dipion contribution to the real photon cross section by 10%-20% (of the ${\rho }^0$ part) and to $\nu W_2$ by several percent. The internal spatial structure of this component is shown to shrink as the photon becomes more virtual. Various views of diffractive pion pair photoproduction are reconciled. The Drell process is interpreted in terms of the dipion component and some evidence is given for its presence in inclusive ${\pi }^{-}$-photoproduction data. Some speculations about the consequences of the spatial size of different photon constituents are given.