Resolved photon processes

Abstract

We review the present level of knowledge of the hadronic structure of the photon, as revealed in interactions involving quarks and gluons "in" the photon. The concept of photon structure functions is introduced in the description of deep-inelastic eγ scattering, and existing parametrizations of the parton densities in the photon are reviewed. We then turn to hard γp and γγ collisions, where we treat the production of jets, heavy quarks, hard (direct) photons, J/ψ mesons, and lepton pairs. We also comment on issues that go beyond perturbation theory, including recent attempts at a comprehensive description of both hard and soft γp and γγ interactions. We conclude with a list of open problems.