A Global Test of QCD Theories for Direct Photon Production

Direct photon production data at fixed target and collider experiments from $pp$ and $\bar{p}p$ collisions are analysed in NLO QCD. The results are grouped into three sets having $\sqrt{s}=20-30$ GeV, 630 GeV and 1.8 TeV. It is seen that at the lowest energies considered, the theory systematically underpredicts the results for all the values of the transverse momentum. At the intermediate energy of the UA2 experiment, the data are more accurately described at high $p_T$, and at the highest collider energies no significant underprediction is seen even at low $p_T$, when using recent structure functions as measured at HERA.