Prompt Photon Production at the Tevatron

In Leading Order (LO) prompt photons originating in the hard interaction between two partons are produced mainly via quark-gluon Compton scattering (qg → qγ) or quark–anti-quark annihilation (qq̄ → gγ). Studies of direct photons with large transverse momenta, pγT , provide precision tests of perturbative QCD (pQCD) as well as information on the distribution of partons within protons, particularly the gluon. These data are also used in global fits of parton distributions functions (PDFs). The LO contributions to di-photon production are quark–anti-quark annihilation (qq̄ → γγ) and gluon-gluon scattering (gg → γγ). The latter subprocess involves initial state gluons coupling to the photons through a quark box; thus the subprocess is suppressed by a factor α2s. The rate is still high for small γγ masses due to the large flux of gluons. Processes where both photons originate from parton fragmentation or where one photon is prompt and one photon is from parton fragmentation also contribute in LO. Di-photon final states are not only interesting to study pQCD but they are also signatures for many new physics processes, such as Higgs production at the LHC or Large Extra Dimensions.