Prompt meson production at large PT

We study the higher twist production mechanism of all s-wave quarkonia as well as the three lowest lying 2-gluon gluonia. After reviewing the basic notions which express the conditions under which perturbative QCD applies to prompt production, we work out all the differential cross sections of hard processes in which, aside from the prompt meson, there are either only partons (q, antiq, g) or partons and a single photon. After pointing out some general remarks about the properties of the higher twist mechanism which follow from these results, we give numerical evaluations of prompt production in itself and compare it with what is expected from the parton fragmentation picture alone. To this end, an extensive use is done of the lund high-PT Monte Carlo as well as its modified versions which include the prompt production mechanism. Interesting channels are quoted, in hadron-hadron and photon-hadron scatterings, for which the higher twist signal is expected to be enhanced compared to the parton fragmentation background