We show that very high momentum transfer semi-inclusive photoproduction of pions from nucleons, γp→πX, is dominated by a subprocess where the pions are produced directly. Although the ‘‘direct subprocess’’ may be thought of as a limiting case of a quark fragmentation subprocess, it is an exclusive process at the quark level, fully calculable using perturbative QCD, and thus distinct from the usual thinking about fragmentation processes. Our numerical results, concentrating on neutral pions, indicate that the direct subprocess is dominant at pion transverse momenta slightly higher than those already measured. Incidentally, the integral over the pion distribution amplitude in the present case is the same as that for the pion electromagnetic form factor, but the momentum transfers involved correspond to measuring the pion form factor at momentum transfers up to several hundred ${\mathrm{GeV}}^2$.

• Received 9 April 1993

DOI:https://doi.org/10.1103/PhysRevD.48.2000