Hard Photoproduction at HERA

In view of possible photoproduction studies in ultraperipheral heavy-ion collisions at the LHC, we briefly review the present theoretical understanding of photons and hard photoproduction processes at HERA, discussing the production of jets, light and heavy hadrons, quarkonia, and prompt photons. We address in particular the extraction of the strong coupling constant from photon structure function and inclusive jet measurements, the infrared safety and computing time of jet definitions, the sensitivity of dijet cross sections on the parton densities in the photon, factorization breaking in diffractive dijet production, the treatment of the heavy-quark mass in charm production, the relevance of the color-octet mechanism for quarkonium production, and isolation criteria for prompt photons. Hard Photoproduction at HERA 2 Figure 1. Factorization of direct (left) and resolved (right) photoproduction in the QCD-improved parton model.