Elastic and transition form factors of light pseudoscalar mesons from QCD sum rules

Original · EN

We revisit Fπ(Q²) and Fₚᵧ(Q²), P=π,η,η', making use of the local-duality (LD) version of QCD sum rules. We give arguments, that the LD sum rule provides reliable predictions for these form factors at Q² ≥ 5-6 GeV², the accuracy of the method increasing with Q² in this region. For the pion elastic form factor, the well-measured data at small Q² give a hint that the LD limit may be reached already at relatively low values of momentum transfers, Q²≈ 4-8 GeV²; we therefore conclude that large deviations from LD in the region Q²=20-50 GeV² seem very unlikely. The data on the (η,η')→γγ* form factors meet the expectations from the LD model. However, the BaBar results for the π⁰→γγ* form factor imply a violation of LD growing with Q² even at Q²≈ 40 GeV², at odds with the η,η' case and with the general properties expected for the LD sum rule.