A comment on the impact of CMD-3 $e^+e^- \to \pi^+\pi^-$ cross section measurement on the SM $g_\mu-2$ value

Abstract

We estimated an impact of the recent CMD-3 measurement of the $e^+e^- \to \pi^+\pi^-$ total cross section at $0.3 < \sqrt{s} < 1.2$ GeV on the leading order hadronic contribution $a_\mu^{had,LO}$ to the muon anomalous magnetic moment $a_\mu = (g_\mu-2)/2$, in presence of comparably precise ISR measurements of the cross section by BaBar and KLOE experiments being in significant tension with the CMD-3. Assuming that all the experiments are affected by yet unidentified systematic effects, to account for the latter, we scaled the experimental uncertainties following the PDG prescription, thus facilitating a consistent joint fit of the world data on the $e^+e^- \to \pi^+\pi^-$ total cross section. The same procedure was applied in all $e^+e^- \to hadrons$ channels contributing to the dispersive estimate of $a_\mu^{had,LO}$. Despite an inclusion of the new CMD-3 $\pi^+\pi^-$ data, our estimate $a_\mu^{had,LO}(e^+e^-) = (696.2 \pm 2.9) \times 10^{-10}$ is consistent with $a_\mu^{had,LO}(e^+e^-)$ values obtained by other authors before publication of the CMD-3 result. Including our $a_\mu^{had,LO}$ value into the SM prediction for $a_\mu$, we obtain $a_\mu^{SM} = (11~659~184 \pm 4_{\mathrm{tot}}) \times 10^{-10}$ which is by $4.7\sigma$ smaller than the world average for the experimental value $a_\mu^{exp} = (11~659~205.9 \pm 2.2) \times 10^{-10}$. We confirm the observation by the CMD-3 authors that their $\sigma(e^+e^- \to \pi^+\pi^-)$ measurement, when taken alone, implies the $a_\mu^{SM}$ prediction consistent with the $a_\mu^{exp}$ at $\sim 1\sigma$ level.Comment: The note contains a comprehensive bibliography on the total e+e- --> hadrons cross section measurements at sqrt(s) < 12 Ge