Puzzles in the hadronic contributions to the muon anomalous magnetic moment

TL;DR

This paper reviews recent progress and ongoing puzzles in calculating the muon's anomalous magnetic moment within the Standard Model, focusing on hadronic contributions and discrepancies among experimental data sets.

Contribution

It provides updated dispersive fits to new experimental data and analyzes tensions between different measurements affecting the muon g-2 calculations.

Findings

Discrepancies exist between CMD-3 and other experiments in the pion form factor data.

Updated fits reveal tensions impacting the hadronic-vacuum-polarization contribution.

The current situation presents unresolved puzzles in the Standard-Model evaluation of a_mu.

Abstract

We summarize recent developments in the Standard-Model evaluation of the anomalous magnetic moment of the muon $a_\mu$, both in the hadronic-light-by-light and hadronic-vacuum-polarization contributions. The current situation for the latter is puzzling as we are confronted with multiple discrepancies that are not yet understood. We present updated fits of a dispersive representation of the pion vector form factor to the new CMD-3 data set and quantify the tensions with the other high-statistics $e^+e^-\to\pi^+\pi^-$ experiments in the contribution to $a_\mu$ in the energy range up to 1 GeV, as well as in the corresponding contribution to the intermediate Euclidean window.