Hadronic vacuum polarization and vector-meson resonance parameters from e+e− → π0γ

We study the reaction e+e− → π0γ based on a dispersive representation of the underlying π0 → γγ∗ transition form factor. As a first application, we evaluate the contribution of the π0γ channel to the hadronic-vacuumpolarization correction to the anomalous magnetic moment of the muon. We find aπ0γμ|≤1.35 GeV = 43.8(6) × 10−11, in line with evaluations from the direct integration of the data. Second, our fit determines the resonance parameters of ω and φ. We observe good agreement with the e+e− → 3π channel, explaining a previous tension in the ω mass between π0γ and 3π by an unphysical phase in the fit function. Combining both channels we find ¯Mω = 782.736(24)MeV and ¯Mφ = 1019.457(20)MeV for themasses including vacuum-polarization corrections. The φ mass agrees perfectly with the PDGaverage, which is dominated by determinations from the K¯ K channel, demonstrating consistency with 3π and π0γ. For the ω mass, our result is consistent but more precise, exacerbating tensions with the ω mass extracted via isospinbreaking effects from the 2π channel