Measurement of the e⁺e⁻ → π⁺π⁻ cross section between 600 and 900 MeV using initial state radiation

Abstract

We extract the e + e − → π + π − 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 π | 2 as well as the contribution of the measured cross section to the leading-order hadronic vacuum polarization contribution to (g −2) μ. We find this value to be a ππ,LO μ (600-900 MeV) = (368.2 ±2.5 stat ±3.3 sys) •10 −10 , which is between the corresponding values using the BaBar or KLOE data.