The associated charged particle multiplicity of high-pT π0 and single-photon events

Abstract The associated charged particle multiplicities of high- p T π 0 and single-photon events were measured at the CERN intersecting storage rings using lead/liquid-argon calorimeters and a scintillation counter array placed around the intersection region. The average multiplicity on the trigger side for the single-photon events was found to be significantly lower than that for the π 0 events. The away-side multiplicity for both π 0 and single-photon events increases with the trigger particle p T , but, at a fixed p T , the direct photon sample was found to have a slightly lower average multiplicity. The differences in the event structure can be explained if a large fraction of the single photons are produced via qg → γ q constituent scattering.