Revisit of the electromagnetic correction to $\tau\to\pi\pi\nu_\tau$ and its implication for muon $g-2$ based on $\tau$ data

Abstract

In this work we focus on the evaluation of the leading-order hadronic vacuum polarization contribution from the $\pi\pi$ channel to the muon anomalous magnetic moment $a_\mu$ by using the experimental $\tau\to\pi\pi\nu_\tau$ data. The isospin breaking corrections play the decisive role in this approach of computing $a_\mu$. One of such important isospin breaking sources is the long-distance electromagnetic correction factor $G_{\rm EM}$ of the $\tau\to\pi\pi\nu_\tau$ process from the real photon radiation. The latter effect can be calculated from the $\tau\to\pi\pi\nu_\tau\gamma$ amplitude, which is revised in this work within the resonance chiral theory by simultaneously including the even-intrinsic-parity and odd-intrinsic-parity resonance operators. We update the determination of the only unknown resonance coupling through the $\omega\to\pi^0\pi^0\gamma$ decay by including contributions from both the vector and scalar resonances. By taking other remaining contributions from the muon $g-2$ White Paper 2025, we further revise the complete value of $a_\mu$, which turns out to deviate from the newest world average result after Fermilab's measurement at the level of 2.7 $\sigma$.