Optimal Renormalization Scale and Scheme for Exclusive Processes

We use the BLM method to x the renormalization scale of the QCD coupling in exclusive hadronic amplitudes such as the pion form factor and the photon-to-pion transition form factor at large momentum transfer. Renormalization-scheme-independent commensurate scale relations are established which connect the hard scattering sub-process amplitudes that control exclusive processes to other QCD observables such a s the heavy quark potential and the electron-positron annihilation cross section. The commensurate scale relation connecting the heavy quark potential, as determined from lattice gauge theory, to the photon-to-pion transition form factor is in excellent agreement with e! 0 edata assuming that the pion distribution amplitude is close to its asymptotic form p 3f x1 , x. We also reproduce the scaling and normalization of the ! + , data at large momentum transfer. Because the renormalization scale is small, we argue that the eeective coupling is nearly constant, thus accounting for the nominal scaling behavior of the data. However, the normal-ization of the space-like pion form factor F Q 2 obtained from electroproduction experiments is somewhat higher than that predicted by the corresponding commensurate scale relation. This discrepancy may be due to systematic errors introduced by the extrapolation of the p ! + n electroproduction data to the pion pole.