The Total and Jet Photoproduction Cross Sections at HERA and Fermilab

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We present results of calculations of the total and jet photoproduction cross sections at HERA and Fermilab energies¹. The calculations take into account the high-energy QCD structure of the photon and are performed for different photon structure functions. We discuss how recent measurements of the total photoproduction cross section at HERA energies² can provide an important information on the low x behavior of the photon structure function and in a more general context, to the nature of strong interactions. In addition, we show that the photoproduction cross section measurements at Fermilab E683 energies could provide a firmer value for pₜmin, the lower bound on the transverse momentum of outgoing jets, which signals the onset of hard scattering. The extrapolation of our cross section to ultra-high energies, of relevance to the cosmic ray physics, gives significant contribution to the ``conventional'' value, but cannot account for the anomalous muon content observed in the cosmic ray air-showers associated with astrophysical point sources⁴.