From Deep Inelastic Scattering to Photoproduction: a Unified Approach

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