Speaker
Description
The evaluation of the hadronic light-by-light contribution to the $(g-2)_{\mu}$ is hindered by our limited knowledge of nonperturbative physics. Different hadronic models have been introduced in the past to estimate its different contributions, where contact with QCD is achieved by fulfilling a varying number of short-distance constraints. It is not clear, however, which are the most relevant constraints to be fulfilled and whether the analytic form of the hadronic models is compatible with them. In particular, the so-called Melnikov-Vainshtein (MV) constraint is thought to have a substantial impact on the HLbL, yet hadronic models do not naturally implement it. In this talk I will study the interplay between resonance exchange and short-distance constraints using a toy model in a 5-dimensional setup. This toy model provides a full-fledged realization of large-$N_c$ QCD with full analytic control over correlators, and helps clarify the subtleties associated with the MV limit. With it, I will also revisit the (dominant) pion and axial exchange contributions to HLbL.