Hadronic vacuum polarization contribution to $a_{\mu }$ from functional methods with strong and electromagnetic isospin breaking

Abstract

We present a continuum-QCD determination of the leading-order hadronic vacuum-polarization contribution to the anomalous magnetic moment of the muon within the Dyson-Schwinger and Bethe-Salpeter equation framework. The calculation incorporates pion back-reaction, a fully dressed quark-photon vertex with a dynamically generated $\rho$-resonance structure in the timelike region, and both strong and electromagnetic isospin breaking treated self-consistently at the quark level. We obtain $a_{\mu }^{\mathrm{HVP},\mathrm{LO}}(u+d+s+c){|}_{\mathrm{ISB}}=709.7\times 10^{-10}$, in good agreement with recent lattice-QCD determinations, and find an isospin-breaking shift of $\Delta a_{\mu }^{\mathrm{HVP},\mathrm{LO}}=4.5\times 10^{-10}$ ($0.6\%$), demonstrating that isospin-breaking effects, while quantitatively modest, are not negligible. Including the bottom-quark contribution and an indicative estimate of the systematic uncertainties, we obtain our final result, $a_{\mu }^{\mathrm{HVP},\mathrm{LO}}(u+d+s+c+b){|}_{\mathrm{ISB}}=(710.0\pm 14.5)\times 10^{-10}$.

Comments: 11 pages, 4 Figures