Complete structure-dependent analysis of the decay P→l+l−

We use the Mellin-Barnes representation in order to improve the theoretical estimate of mass corrections to the width of light pseudoscalar meson decays into a lepton pair, P→l+l−. The full resummation of the terms (M2/Λ2)n, (m2/M2)n and (m2/Λ2)n to the decay amplitude is performed, where m is the lepton mass, M is the meson mass and Λ≈mρ is the characteristic scale of the P→γ*γ* form factor. The total effect of mass corrections is quite important for η (η′) decays. We also comment on the estimation of the hadronic light-by-light scattering contribution to the muon anomalous magnetic moment in the chiral perturbation theory.