Measurement of the ${\mathrm{e}}^{+}{\mathrm{e}}^{-}\to \ddot{\mathrm{I}}{€}^{+}\ddot{\mathrm{I}}{€}^{-}$ Cross Section between 600 and 900 MeV Using Initial State Radiation

Abstract

We extract the $e^{+}{e}^{-}\to \ddot{I}{€}^{+}\ddot{I}{€}^{-}$ cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb${}^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor $|F_{\ddot{I}}€{|}^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2{)}_{\hat{I}}¼$. We find this value to be $a_{\hat{I}}{¼}^{\ddot{I}€\ddot{I}€,\mathrm{LO}}(600-900\mathrm{MeV})=(368.2\pm 2.5_{\mathrm{stat}}\pm 3.3_{\mathrm{sys}})\cdot 10^{-10}$.

14 pages, 7 figures, accepted by PLB