Unbiased analysis of CLEO data at NLO and the pion distribution amplitude

We discuss different QCD approaches to calculate the form factor F(gamma*)gammapi(Q(2)) of the gamma(*)gamma-->pi(0) transition giving preference to the light-cone QCD sum rules (LCSR) approach as being the most adequate. In this context we revise the previous analysis of the CLEO experimental data on F(gamma*)gammapi(Q(2)) by Schmedding and Yakovlev. Special attention is paid to the sensitivity of the results to the (strong radiative) alpha(s) corrections and to the value of the twist-four coupling delta(2). We present a full analysis of the CLEO data at the NLO level of LCSRs, focusing particular attention on the extraction of the relevant parameters to determine the pion distribution amplitude, i.e., the Gegenbauer coefficients a(2), a(4). Our analysis confirms our previous results and also the main findings of Schmedding and Yakovlev: both the asymptotic as well as the Chernyak-Zhitnitsky pion distribution amplitudes are completely excluded by the CLEO data. A novelty of our approach is to use the CLEO data as a means of determining the value of the QCD vacuum nonlocality parameter lambda(q)(2)=<(q) over barD(2)q>/<(q) over barq>=0.4 GeV2, which specifies the average virtuality of the vacuum quarks.