LIGHT-CONE VIEW ON THE APPLICABILITY OF PERTURBATIVE QCD FOR EXCLUSIVE PROCESSES

It is pointed out in the example of the pion form factor that the usua l factorized hard scattering amplitude T-H in perturbative QCD is deri ved from the light-cone time-ordered perturbative expansion. In the li ght-cone perturbative expansion, the natural variable to make a separa tion of perturbative contributions from contributions intrinsic to the bound-state wave function itself is the light-cone energy, rather tha n the gluon virtuality of T-H. We find that the ''legal'' PQCD contrib ution defined by the light-cone energy cut saturates to the full PQCD prediction without any cut in the smaller Q(2) region as compared to t hat defined by the gluon four-momentum square cut. This is due to the contribution from highly off-energy-shell gluons in the end-point regi ons of the phase space.