Accompanied versus unaccompanied prompt photons

We point out that the usual quantum-chromodynamics (QCD) analysis of data on the production of direct photons in hadron collisions in terms of (quark + gluon) → (quark + photon) Compton scattering breaks down when $x_T(=\frac{2p_T}{\sqrt{s}})$ becomes large (e.g., $p_T\gtrsim 10$ GeV at $\sqrt{s}=63\mathrm{}$ GeV). The bremsstrahlung of photons by quarks produced with large transverse momentum eventually dominates the lowest-order graphs. We speculate that this trend is already present in recent measurements of hadrons produced in association with direct photons. Studying the interplay of the QCD and bremsstrahlung mechanisms greatly enhances the scope as well as the sensitivity of QCD tests. The data at large $x_T$ can directly probe the scaling violations of the gluon structure function, the three-gluon vertex, and the photon structure function.