Measurement of direct photon production at large transverse momentum inπ−p,π+p, andppcollisions at 300 GeV/c

Cross sections for inclusive direct photon production in ${\pi }^{\mathrm{-}}$p, ${\pi }^{+}$p, and pp collisions at 300 GeV/c are measured at transverse momenta $p_T$ up to 7 GeV/c ($x_T$=0.6). For ${\pi }^{\mathrm{-}}$p→γX also the rapidity distribution is presented. The cross-section ratio σ(${\pi }^{\mathrm{-}}$p→γX)/σ(${\pi }^{+}$p→γX) is found to be 1 at $p_T$=4 GeV/c and rises with increasing $p_T$. This observation signals the occurrence of valence-quark–antiquark annihilation. The results are in good agreement with QCD predictions.