Hadronic Light-By-Light Scattering Contribution to the Muong−2: An Effective Field Theory Approach

The hadronic light-by-light contribution to $a_{\mu }$, the anomalous magnetic moment of the muon, is discussed from the point of view of an effective low-energy theory. As an application, the coefficient of the leading logarithm arising from the two-loop graphs involving two anomalous vertices is computed, and found to be positive. This corresponds to a positive sign for the pion-pole contribution to the hadronic light-by-light correction to $a_{\mu }$, and to a sizable reduction of the discrepancy between the present experimental value of $a_{\mu }$ and its theoretical counterpart in the standard model.