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 usual factorized hard scattering amplitude TH in perturbative QCD is derived from the light-cone time-ordered perturbative expansion. In the light-cone perturbative expansion, the natural variable to make a separation of perturbative contributions from contributions intrinsic to the bound-state wave function itself is the light-cone energy, rather than the gluon virtuality of TH. We find that the ``legal'' PQCD contribution defined by the light-cone energy cut saturates to the full PQCD prediction without any cut in the smaller Q² region as compared to that defined by the gluon four-momentum square cut. This is due to the contribution from highly off-energy-shell gluons in the end-point regions of the phase space.