Measurement of the e+ e- → π+ π- π0 cross section in the energy range 0.62-3.50 GeV at Belle II

We report a measurement of the e(+) e(-) -> pi(+) pi(-) pi(0) 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 Upsilon(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(-) -> pi(+) pi(-) pi(0) gamma events, which involve a pi(0) -> gamma gamma 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 omega and phi resonances is dominated by the systematic uncertainty of 2.2%. The resulting cross sections in the 0.62-1.80 GeV energy range yield a(mu)(3 pi) = [48.91 +/- 0.23(stat) +/- 1.07(syst) x 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.