From deep inelastic scattering to photoproduction: A unified approach

The strikingly different high energy behaviors of real photoabsorption cross sections with $Q^2=0\mathrm{}$ and the low $x$ proton structure function at large $Q^2$ are studied from a laboratory frame viewpoint, in which the $x$ and $Q^2$ dependence reflects the space-time structure of the interaction. This is done using a simple model which incorporates hadron dominance, but attributes the striking enhancement observed at DESY HERA at very low $x$ and high $Q^2$ to contributions from heavy long-lived fluctuations of the incoming photon. Earlier published predictions of the model for the then unknown behavior of the structure function at small $x$ and intermediate $Q^2$ are shown to be strikingly confirmed by recent experimental data. A simultaneous analysis of real photoabsorption data and structure function data for $0<~x<\mathrm{}0.1\mathrm{}$ and $0<~Q^2<~15$ GeV${}^2$ is then reported. An excellent fit is obtained, with all parameters in the restricted ranges allowed by other physical requirements.