Precision measurement of $\sigma (e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\gamma )/\sigma (e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}\gamma )$ and determination of the ${\pi }^{+}{\pi }^{-}$ contribution to the muon anomaly with the KLOE detector

Abstract

We have measured the ratio $\sigma (e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\gamma )/\sigma (e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}\gamma )$, with the KLOE detector at DA$\Phi$NE for a total integrated luminosity of $\sim$ 240 pb${}^{-1}$. From this ratio we obtain the cross section $\sigma (e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-})$. From the cross section we determine the pion form factor $|F_{\pi }{|}^2$ and the two-pion contribution to the muon anomaly $a_{\mu }$ for $0.592<M_{\pi \pi }<0.975$ GeV, ${\Delta }^{\pi \pi }{a}_{\mu }$= $(385.1\pm 1.1_{\mathrm{stat}}\pm 2.7_{\mathrm{sys}+\mathrm{theo}})\times 10^{-10}$. This result confirms the current discrepancy between the Standard Model calculation and the experimental measurement of the muon anomaly.

Comments: 18 pages, 8 figures, minor text corrections, one table added, version to appear on Physics Letters B