Measurement of the $e^+e^- \to π^+π^-π^0$ cross section in the energy range 0.62-3.50 GeV at Belle II

We report a measurement of the e+e− → π+π−π0 cross section in the energy range from 0.62 to 3.50 GeV using an initial-state radiation technique. We use an e+e− data sample corresponding to 191 fb−1 of integrated luminosity, collected at a center-of-mass energy at or near the ϒ(4S) resonance with the Belle II detector at the SuperKEKB collider. Signal yields are extracted by fitting the two-photon mass distribution in e+e− → π+π−π0γ events, which involve a π0 → γγ decay and an energetic photon radiated from the initial state. Signal efficiency corrections with an accuracy of 1.6% are obtained from several control data samples. The uncertainty on the cross section at the ω and ϕ resonances is dominated by the systematic uncertainty of 2.2%. The resulting cross sections in the 0.62–1.80 GeV energy range yield a3πμ = [48.91 ± 0.23(stat) ± 1.07(syst)] × 10−10 for the leading-order hadronic vacuum polarization contribution to the muon anomalous magnetic moment. This result differs by 2.5 standard deviations from the most precise current determination.