The Hadronic Contribution to the Muon Anomalous Magnetic Moment and to the Running Electromagnetic Fine Structure Constant at MZ - Overview and Latest Results

TL;DR

This paper reviews the Standard Model prediction of the muon anomalous magnetic moment, focusing on the hadronic contributions, and discusses recent results and their implications for potential new physics.

Contribution

It provides an updated overview of the hadronic contributions to the muon g-2 and the running electromagnetic coupling at MZ, incorporating latest experimental data and analysis methods.

Findings

Discrepancies of 3.6 sigma and 2.4 sigma between predictions and experiment for a_mu from e+e- and tau data.

The dominant hadronic contribution arises from the two-pion channel.

Latest results impact the precision of the electromagnetic coupling at the Z mass.

Abstract

Quantum loops induce an anomaly, a_mu, 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 tau decays. The corresponding e+e- and tau-based predictions of a_mu exhibit deviations by, respectively, 3.6 sigma and 2.4 sigma 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.