The muon $g-2$ and $\alpha(M_Z^2)$: a new data-based analysis

TL;DR

This paper provides a comprehensive re-evaluation of hadronic contributions to the muon g-2 and the effective QED coupling at the Z boson mass, using a new data combination method that improves accuracy and consistency.

Contribution

It introduces a novel bias-free data combination method for $e^+e^- ightarrow { m hadrons}$ cross sections, leading to more precise estimates of hadronic vacuum polarization effects.

Findings

The hadronic vacuum polarization contribution to muon g-2 is estimated as (693.27 ± 2.46)×10^{-10}.

The Standard Model prediction for muon g-2 is 3.7σ below the experimental value.

The hadronic contribution to the QED coupling at the Z boson mass is (276.11 ± 1.11)×10^{-4}.

Abstract

This work presents a complete re-evaluation of the hadronic vacuum polarisation contributions to the anomalous magnetic moment of the muon, $a_{\mu}^{\rm had, \, VP}$ and the hadronic contributions to the effective QED coupling at the mass of the $Z$ boson, $\Delta\alpha_{\rm had}(M_Z^2)$, from the combination of $e^+e^-\rightarrow {\rm hadrons}$ cross section data. Focus has been placed on the development of a new data combination method, which fully incorporates all correlated statistical and systematic uncertainties in a bias free approach. All available $e^+e^-\rightarrow {\rm hadrons}$ cross section data have been analysed and included, where the new data compilation has yielded the full hadronic $R$-ratio and its covariance matrix in the energy range $m_{\pi}\leq\sqrt{s}\leq 11.2$ GeV. Using these combined data and pQCD above that range results in estimates of the hadronic vacuum polarisation contributions to $g-2$ of the muon of $a_{\mu}^{\rm had, \, LO \, VP} = (693.27 \pm 2.46)\times 10^{-10}$ and $a_{\mu}^{\rm had, \, NLO \, VP} = (-9.82 \pm 0.04)\times 10^{-10}$. The new estimate for the Standard Model prediction is found to be $a_{\mu}^{\rm SM} = (11\ 659 \ 182.05 \pm 3.56) \times 10^{-10}$, which is $3.7\sigma$ below the current experimental measurement. The prediction for the five-flavour hadronic contribution to the QED coupling at the $Z$ boson mass is $\Delta\alpha_{\rm had}^{(5)}(M_Z^2)= (276.11 \pm 1.11)\times 10^{-4}$, resulting in $\alpha^{-1}(M_Z^2) = 128.946 \pm 0.015$. Detailed comparisons with results from similar related works are given.