Observed differences in event structures of high-p/sub T/ pi /sup 0/ and single photon events produced in pp collisions in the CERN ISR

The direct photon production in pp collisions for c.m. energies 31<or= square root s<or=63 GeV and transverse momenta of up to 9 GeV/c were measured at the ISR using a segmented lead/liquid argon calorimeter. The observed gamma / pi /sup 0/ ratio is found to be significantly larger than zero at 4 GeV/c in p/sub T/ increasing to 0.4 at 9 GeV/c. The average multiplicity on the trigger side for the single-photon events was found to be significantly lower than for pi /sup 0/ events. The correlations in Delta y and Delta phi between the trigger particle and an additional particle are found to differ mainly at small Delta y and Delta phi . (14 refs).