Kavli Affiliate: T. Higuchi
| First 5 Authors: Belle II Collaboration, I. Adachi, L. Aggarwal, H. Aihara, N. Akopov
| Summary:
We report a measurement of the $e^+e^- to pi^+pi^-pi^0$ 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 $text{fb}^{-1}$
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 $e^+e^- to pi^+pi^-pi^0gamma$ events, which involve a
$pi^0 to gammagamma$ 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^{3pi} = [48.91 pm 0.23~(mathrm{stat}) pm
1.07~(mathrm{syst})] times 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.
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