What is this thing called pion distribution amplitude? From theory to data

We discuss the status of the pion distribution amplitude (DA) from analyzing the CLEO experimental data in the context of QCD sum-rule techniques and QCD perturbation theory at the NLO accuracy. The constraints extracted this way for the Gegenbauer coefficients a_2 and a_4 exclude \Phi_{CZ} at the 4\sigma level, while \Phi_{asy} is outside the 3\sigma error ellipse. These data provide strong support for the type of endpoint-suppressed, double-humped pion DA we derived via QCD sum rules with nonlocal condensates and favor a value of the vacuum quark virtuality \lambda_q^2 \simeq 0.4 GeV^2. This pion DA is in agreement with the E791 data, though these experimental results should be viewed carefully and further confirmation is necessary for a more accurate judging of pion DAs from them.