Inclusive ${\pi }^0$ and direct-photon cross sections in the kinematic range $3.5<\mathrm{}{p}_T<12\mathrm{}\mathrm{GeV}/c$ with central rapidities ( $y_{\mathrm{cm}}$) are presented for $530\mathrm{and}800\mathrm{GeV}/c$ proton beams and a $515\mathrm{GeV}/c$ ${\pi }^{-}$ beam incident on Be targets. Current next-to-leading-order perturbative QCD calculations fail to adequately describe the data for conventional choices of scales. Kinematic distributions from these hard scattering events provide evidence that the interacting partons carry significant initial-state parton transverse momentum ( $k_T$). Incorporating these $k_T$ effects phenomenologically greatly improves the agreement between calculations and the measured cross sections.

• Received 24 November 1997

DOI:https://doi.org/10.1103/PhysRevLett.81.2642