Probing Partonic Structure inγ*γ→ππnear Threshold

Hadron pair production ${\gamma }^{*}\gamma \to h\overline{h}$ in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new nonperturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where $h$ is a pion is emphasized.