Measurement of the e⁺e⁻ → π⁺π⁻ Cross Section between 600 and 900 MeV Using Initial State Radiation

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 |Fπ|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(μ)^{ππ,LO} (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.