Renormalization-group improved evolution of the meson distribution amplitude at the two-loop level

We discuss the two-loop evolution of the flavor-nonsinglet meson distribution amplitude in perturbative QCD. After reviewing previous two-loop computations, we outline the incompatibility of these solutions with the group property of the renormalization-group transformations. To cure this deficiency, we compute a correction factor for the nondiagonal part of the meson evolution equation and prove that with this modification the two-loop solution conforms with the group properties of the renormalization-group transformations. The special case of a fixed strong coupling (no Q2 dependence) is also discussed and comparison is given to previously obtained results.