A UNITARITY BOUND AND THE COMPONENTS OF PHOTON-PROTON INTERACTIONS

We show how and why the short distance (''hard'') interaction, which i s calculated in perturbative QCD, provides a mass cutoff in Gribov's f ormula for photon-proton collisions. This enables us to find a new and more restrictive unitarity bound for this process, sigma(gammap) les s than or equal to C(ln 1/x)(5/2). We develop a simple model that cons ists of ''soft'' and ''hard'' contributions, which yields a qualitativ e description of the published experimental data over a wide range of photon virtualities (Q(2)) and energies (W). This model provides a qua ntitative way of evaluating the relative rate of the short and long di stance contributions, in the different kinematic regions. The main res ults of the analysis are (i) that even at Q(2) = 0 and high energies t he short distance contribution is not small, and it provides a possibl e explanation for the experimental rise of the photoproduction cross s ection; and (ii) at large values of Q(2), the long distance processes still contribute to the total cross section.