The hadronic contribution to the muon anomalous magnetic moment and to the running electromagnetic fine structure constant at MZ — overview and latest results

Quantum loops induce an anomaly, aμ, in the magnetic moment of the muon that can be accurately measured. Its Standard Model prediction is limited in precision by contributions from hadronic vacuum polarisation of the photon. The dominant lowest-order hadronic term can be calculated with a combination of experimental cross section data, involving e+e− annihilation to hadrons, and perturbative QCD. These are used to evaluate an energy-squared dispersion integral that strongly emphasises low photon virtualities. The dominant contribution to the integral stems from the two-pion channel that can be measured both in e+e− annihilation and in τ decays. The corresponding e+e− and τ-based predictions of aμ exhibit deviations by, respectively, 3.6σ and 2.4σ from experiment, leaving room for a possible interpretation in terms of new physics. This talk reviews the status of the Standard Model prediction with emphasis on the lowest-order hadronic contribution. Also given is the latest result for the running electromagnetic fine structure constant at the Z-mass pole, whose precision is limited by hadronic vacuum polarisation contributions, determined in a way similar to those of the magnetic anomaly.