Large Nc in chiral perturbation theory

The construction of the effective Lagrangian relevant for the mesonic sector of QCD in the large N-c limit meets with a few rather subtle problems. We thoroughly examine these and show that, if the variables of the effective theory are chosen suitable, the known large N-c counting rules of QCD can unambiguously be translated into corresponding counting rules for the effective coupling constants. As an application, we demonstrate that the Kaplan-Manohar transformation is in conflict with these rules and is suppressed to all orders in 1/N-c. The anomalous dimension of the axial singlet current generates an additional complication: The corresponding external field undergoes nonmultiplicative renormalization. As a consequence, the Wess-Zumino-Witten term, which accounts for the U(3)(R) x U(3)(L) anomalies in the frameworth of the effective theory, contains pieces that depend on the running scale of QCD. The effect only shows up at nonleading order in 1/N-c, but requires specific unnatural parity contributions in the effective Lagrangian that restore renormalization group invariance.