Abstract
We fitted the ππ mass distribution in the range 0.5≤ M ππ ≤1.1 GeV measured in hard exclusive positron–proton reactions at HERA by the form dictated by QCD at leading twist level. Extracted parameters are related to valence quark distribution in the pion, and to the pion and ρ meson distribution amplitudes. We obtain, for the first time, a measurement of the second Gegenbauer coefficient of the ρ meson distribution amplitude: a 2 ( ρ) =−0.10±0.20 for a photon virtuality of 〈 Q 2〉=21.2 GeV 2.
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