  We compute all kaon and pion parton distribution amplitudes (PDAs) to
twist-three and find that only the pseudotensor PDA can reasonably be
approximated by its conformal limit expression. At terrestrially accessible
energy scales, the twist-two and pseudoscalar twist-three PDAs differ
significantly from those functions commonly associated with their forms in
QCD's conformal limit. In all amplitudes studied, SU(3) flavour-symmetry
breaking is typically a 13% effect. This scale is determined by nonperturbative
dynamics; namely, the current-quark-mass dependence of dynamical chiral
symmetry breaking. The heavier-quark is favoured by this distortion, for
example, support is shifted to the s-quark in the negative kaon. It appears,
therefore, that at energy scales accessible with existing and foreseeable
facilities, one may obtain reliable expectations for experimental outcomes by
using these "strongly dressed" PDAs in formulae for hard exclusive processes.
Following this procedure, any discrepancies between experiment and theory will
be significantly smaller than those produced by using the conformal-limit PDAs.
Moreover, the magnitude of any disagreement will either be a better estimate of
higher-order, higher-twist effects or provide more realistic constraints on the
Standard Model.
