Hadron production near threshold in photon-photon collisions.

A dual picture between perturbative QCD and resonance production is exploited to describe non-diffractive meson pair production near threshold in 77 annihilation. Submitted to Physical Review Letters * Work supported in part by the Department of Energy, contract DE-AC03-76SF00515. In quantum chromodynamics, hadron production in 77 annihilation is initiated by the coupling of the incident photons to either one or two quark pairs. At high momentum transfer, both inclusive and exclusive reactions can be computed in terms of underlying elementary quark and gluon scattering subprocesses.’ A large body of data from PEP and PETRA gives good support to the validity of this perturbative QCD description. At low momentum transfer, the processes which convert the quarks into hadrons are generally nonperturbative, coherent, complex, and thus, at this point, not calculable. There are, however, exceptional low momentum processes where perturbative QCD predictions should be reliable even at threshold energies if Coulombic rescattering corrections are properly taken into account; e.g., those rare 77 annihilation processes in which only hadrons containing heavy quarks are produced. As an example, we illustrate the leading order minimally-connected QCD diagrams for heavy meson pair, production in Fig. 1. Since the exchanged gluon momenta and the effective renormalization scale are of order of the heavy quark mass a perturbative QCD analysis can be justified. Unfortunately, the predicted rate is small, and tests of this basic feature of QCD appear remote. Nevertheless, we shall argue here, that given the validity of a dual picture between the perturbative amplitudes and suitably-averaged resonant contributions, one can obtain reasonable estimates of relatively light meson pair production 77 cross sections even near threshold. Let us recall the situation for heavy quark-pair production in e+eannihilation. In this case the cross section is well under control in QCD not only at high energies but also near threshold where only a minimal number of hadrons