Inclusive direct photon invariant cross sections have been measured in both pp and pp collisions at √s = 24.3 GeV at the CERN SPS, permitting the first measurement of the difference of the pp and pp cross sections. The direct photon cross section in pp collisions has been found to be systematically larger than that in pp collisions, which indicates a significant contribution of the qq annihilation term as predicted by theoretical calculations.
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We would like to thank the CERN staff, particularly those in the SPS and CN divisions, without whom this experiment would not have been possible. The following funding agencies provided support: the Swiss National Funding Agency, the US Department of Energy, and the US National Science Foundation. We are grateful for the continued efforts of our secretarial and technical staff.