Pion Elastic and pi to gamma gamma* Transition Form Factors at Large Momentum Transfers

Abstract

We present the results of our recent analyses of the form factors F_pi(Q^2) and F_{P gamma}(Q^2), P = pi, eta, eta', within the local-duality version of QCD sum rules. To probe the expected accuracy of this method, we consider, in parallel to QCD, a quantum-mechanical potential model. In the latter case, the exact form factor may be calculated from the solutions of the Schr\"odinger equation and confronted with the result from the quantum-mechanical local-duality sum rule. We find that the local-duality sum rule is expected to yield reliable predictions for both F_pi(Q^2) and F_{pi gamma}(Q^2) in the region Q^2 > 5-6 GeV^2. Moreover, in this region the accuracy of this approach improves rather fast with increasing Q^2. For the elastic form factor F_pi(Q^2), we are therefore forced to conclude that large deviations from the local-duality limit in the region Q^2 = 20-50 GeV^2 reported in some recent theoretical studies seem to us unlikely. The data on the eta, eta' to gamma gamma* transition form factors meet pretty well the predictions of a "local-duality model." Interestingly, recent results for the pi to gamma gamma* transition form factor hint at a violation of local duality rising with Q^2; this is at odds with the eta and eta' cases and all our experience from quantum mechanics.