Article révisé par les pairs
Résumé : 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. © 2000 Elsevier Science B.V.