QCD rum rules: Form factors and wave functions

The shape of hadronic distribution amplitudes (DAs) is a critical issue for the perturbative QCD of hard exclusive processes. Recent CLEO data on γγ* → π0 form factor clearly favor a pion DA close to the asymptotic form. We argue that QCD sum rules for the moments of the pion DA φπ(x) are unreliable, so that the humpy shape of φπ(x) obtained by Chernyak and Zhitnitsky is a result of model assumptions rather than an unambiguous consequence of QCD sum rules. This conclusion is also supported by a direct QCD sum rule calculation of the γγ* → π0 form factor which gives a result very close to the CLEO data. (author)