Testing Quantum Chromodynamics in Anti-proton Reactions*

An experimental program with anti-protons at intermediate energy can serve as an important testing ground for QCD. Detailed predictions for exclusive cross sections at large momentum transfer based on perturbative QCD and the QCD sum rule form of the proton distribution amplitude are available for j?p + y-y -for both real and virtual photons. Meson-pair and lepton-pair final states also . . give sensitive tests of the theory. The production of charmed hadrons in exclusive up channels may have a non-negligible cross section. Anti-proton interactions in a nucleus, particularly J/$ production, can play an important role in clarifying fundamental QCD issues such as color transparency, critical length phenomena, and the validity of the reduced nuclear amplitude phenomenology. Presented at The IVth LEAR Workshop Villars-Sur-Ollon, Switzerland, September 6-13, 1987. * Work supported by the Department of Energy, contract DE-AC03-76SF00515. INTRODUCTION Quantitative tests of Quantum Chromodynamics generally involve high momentum transfer where factorization theorems and asymptotic freedom allow detailed predictions based on perturbative quark and gluon subprocesses. The most challenging testing ground of the theory is now the intermediate (few GeV/c) momentum transfer domain where both perturbative and non-perturbative aspects of the theory are manifest. In this talk, I will focus on a class of exclusive and inclusive antiproton reactions which can test important and novel features of QCD even at moderate energy. Further discussion may be found in several recent reports (Brodsky, 1986 and 1987). EXCLUSIVE PROCESSES One of the most elegant applications of QCD is to exclusive processes at large momentum transfer such as j~p + AL? where A and B can be photons, leptons, or hadrons. Such reactions can be factorized (Lepage and Brodsky, 1980; Brodsky, et.al., 1980; Efremov and Radyushkin, 1980; Duncan and Mueller, 1980; Chernyak .and Zhitnitskii, 1984) into a convolution of factors: the distribution amplitudes $H(x, Q) which contain the non-perturbative dynamics of each incident and outgoing hadron multiplied by a perturbatively-calculable amplitude for the scattering of the quarks from the incident to final direction. The logarithmic dependence of the distribution amplitudes is controlled in leading order by gluon exchange and can be derived from evolution equations or renormalization group methods. In first approximation, one derives fixed angle scaling laws (Sivers, et. a1.,1976), daldt = f (O,,)/S~-~, where according to QCD quark counting rules, (Brodsky and Farrar, 1973; Matveev, et. al. 1973) N is the total number of incident and final fields. In the case of pp + yy and e+ethe explicit dependence of the angular function f (t?,,) has b een worked out in detail. (See below.) In general, the angular dependence reflects the underlying duality graph (minimally-connected quark-gluon subprocess amplitudes). In some diagrams, pinch singularities arise fLandshoff,1974) h w ere propagators can become nearly-on-shell, but this region is suppressed by Sudakov form factors (Mueller, 1981). This effect leads in some