Direct photon production by positive- and negative-pion beams

Large-$p_T$ direct photon production by ${\pi }^{\mp }$ beams in the framework of QCD is studied in detail. Higher-order corrections (K factors), photon bremsstrahlung, partons’ intrinsic transverse momentum, and higher-twist effects are taken into account. The various ambiguities which beset a theoretical calculation are carefully considered. Comparisons with all available data and further predictions are presented in detail. At fixed energy the ratio of the inclusive cross sections by ${\pi }^{\mathrm{-}}$ and ${\pi }^{+}$ shows a clear increase with $p_T$. Certain preliminary data do not support this prediction. It is concluded that precise measurements of this ratio offer an important test of QCD.