Probing partonic structure in γ ∗ γ → ππ near threshold

Abstract

Hadron pair production γ ∗ γ → h ¯ 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 non-perturbative 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. 1 Introduction. Hadron production at small invariant mass or even near threshold is usually quite inaccessible to a partonic description. We advocate that a notable exception is provided by the process γ ∗ γ → h ¯ h with a highly virtual photon, more precisely in the regime where the squared c.m. energy W 2 is much smaller than the photon virtuality Q 2. We will here focus on pairs of spin zero mesons, e.g. pions or kaons, but our analysis can