Leading-twist distribution amplitudes (DAs) of light mesons like pi,rho etc.
describe the leading nonperturbative hadronic contributions to exclusive QCD
reactions at large energy transfer, for instance electromagnetic form factors.
They also enter B decay amplitudes described in QCD factorisation, in
particular nonleptonic two-body decays. Being nonperturbative quantities, DAs
cannot be calculated from first principles, but have to be described by models.
Most models for DAs rely on a fixed order conformal expansion, which is
strictly valid for large factorisation scales, but not always sufficient in
phenomenological applications. We derive models for DAs that are valid to all
orders in the conformal expansion and characterised by a small number of
parameters which are related to experimental observables.Comment: 19 pages, 10 figure
