πN transition distribution amplitudes: Their symmetries and constraints from chiral dynamics

Baryon to meson transition distribution amplitudes (TDAs) extend the concept of generalized parton distributions. Baryon to meson TDAs appear as building blocks in the collinear factorized description of amplitudes for a class of hard exclusive reactions, prominent examples of which being hard exclusive meson electroproduction off a nucleon in the backward region and baryon-antibaryon annihilation into a meson and a lepton pair. We study the general properties of these objects following the underlying symmetries of QCD. In particular, the Lorentz symmetry results in the polynomiality property of the Mellin moments in longitudinal momentum fractions. We present a detailed account of the isotopic and permutation symmetry properties of nucleon to pion (pi N) TDAs. This restricts the number of independent leading twist pi N TDAs to eight functions, providing description of all isotopic channels. Using chiral symmetry and the crossing relation between pi N TDAs and pi N generalized distribution amplitudes, we establish soft pion theorems for pi N TDAs, which determine the magnitude of pi N TDAs. Finally, we build a simple resonance exchange model for pi N TDAs considering N and Delta(1232) exchange contributions into the isospin-1/2 and isospin-3/2 pi N TDAs.