Production of High Mass Pairs of Direct Photons and Neutral Mesons in a Tevatron Fixed Target Experiment

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The production of high-mass pairs of direct photons, $\pi^0$'s, and $\eta$'s has been measured by Fermilab experiment E706. The experimental apparatus included a large,finely segmented lead{liquid argon electromagnetic calorimeter and a charged particle spectrometer consisting of silicon microstrip detectors in the target region and multiwire proportional chambers and straw tube drift chambers downstream of an analysis magnet. The experiment triggered on localized high-$p_T$ depositions in the electromagnetic calorimeter; the high-mass pair data required two depositions on opposite sides of the calorimeter. Correlations between high $p_T$ particles are used to extract information about the transverse momentum of partons ($k_T$). Comparisons are made between the diphoton data and the results of next-to-leading order perturbative Quantum Chromodynamic (NLO pQCD) calculations. The shapes of the NLO pQCD results are inconsistent with the data distributions. A resummed NLO pQCD calculation, which incorporates the effects of multiple soft-gluon emission, provides reasonable matches to the shapes of the data distributions. Similar distributions of$ \pi^{0}\pi^{0}$ and $\gamma\pi^{0}$ pairs are compared with leading order pQCD calculations which approximates initial-state $k_T$ effects by a Gaussian smearing technique. These calculations, using values consistent with the diphoton data, successfully reproduce the shapes of the data distributions. The theory can provide a good representation of $k_T$-insensitive distributions, such as the mass of the pair and particle $p_T$. Results from the high-mass pair data are used as input to a phenomenological $k_T$-smearing model which provides a consistent description of the observed deviations of NLO pQCD calculations from the inclusive direct-photon and $\pi^0$ data.