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Measurement of the $e^+e^- \to π^+π^-π^0$ cross section in the energy range 0.62-3.50 GeV at Belle II (2404.04915v2)

Published 7 Apr 2024 in hep-ex

Abstract: We report a measurement of the $e+e- \to \pi+\pi-\pi0$ 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 $e+e- \to \pi+\pi-\pi0\gamma$ events, which involve a $\pi0 \to \gamma\gamma$ 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 \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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