The Discrepancy Between tau and e+e- Spectral Functions Revisited and the Consequences for the Muon Magnetic Anomaly

TL;DR

This paper revisits the comparison of spectral functions from tau decays and e+e- annihilation, applying new corrections to improve agreement and refine the muon g-2 theoretical estimate, reducing discrepancies with experimental data.

Contribution

It provides an updated analysis of isospin-breaking corrections and their impact on the muon magnetic anomaly calculation using tau and e+e- data.

Findings

Improved agreement between tau decay and e+e- spectral functions.

Reduced difference between tau-based and direct muon g-2 measurements.

Muon magnetic anomaly estimate is 1.9 standard deviations below the experimental value.

Abstract

We revisit the procedure for comparing the pi pi spectral function measured in tau decays to that obtained in e+e- annihilation. We re-examine the isospin-breaking corrections using new experimental and theoretical input, and find improved agreement between the tau- --> pi- pi0 nu_tau branching fraction measurement and its prediction using the isospin-breaking-corrected e+e- --> pi+pi- spectral function, though not resolving all discrepancies. We recompute the lowest order hadronic contributions to the muon g-2 using e+e- and tau data with the new corrections, and find a reduced difference between the two evaluations. The new tau-based estimate of the muon magnetic anomaly is found to be 1.9 standard deviations lower than the direct measurement.