Direct photons have been studied in pp̄ interactions at √s = 24.3 GeV and in the transverse momentum (pT) range 3-7 GeV/c(0.25 < xT < 0.58). The experiment was performed using an internal H2 cluster the target in the CERN pp̄ Collider. The measured invariant cross section is compared with recent theoretical predictions.
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Direct photons are expected to be produced in hadronic collisions ~ through three processes: gluon Compton scattering, quark-antiquark annihilations, and bremsstrahlung offa quark line. The bremsstrahlung contribution has been shown to be small [ 2 ]. In pp collisions, where direct photons were first observed [ 3 ], the main contribution has been shown to come from gluon Compton scattering [4,5 ]. This is explained by the absence of valence antiquarks in the proton, which are necessary for the annihilation mechanism. Valence antiquarks in n-, n+ and 15 allow an appreciable annihilation contribution in collisions of these particles with protons. Several experiments involving pions have been performed Permanent address: Fermilab, Batavia, IL 60510, USA. 2 Also at INFN, 1-40126 Bologna, Italy. 3 Work supported by the Swiss National Science Foundation. 4 Work supported by the US Department of Energy. 5 Work supported by the US National Science Foundation. ~t See ref. [ 1 ] for a review.