Partonic structure of pi and rho mesons from data on hard exclusive production of two pious off nucleon

We fitted the pi pi mass distribution in the range 0.5 less than or equal t o M-pi pi less than or equal to 1.1 GeV measured in hard exclusive positron -proton reactions at HERA by the form dictated by QCD at leading-twist leve l. Extracted parameters are related to valence-quark distribution in the pi on, and to the pion and rho meson distribution amplitudes. We obtain, for t he first time, a measurement of the second Gegenbauer coefficient of the rh o meson distribution amplitude: a(2)((rho)) = -0.10 +/- 0.20 for a photon v irtuality of (Q(2)) = 21.2 GeV2. (C) 2000 Elsevier Science B.V. All rights reserved.