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−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−12.93 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|Fπ|2 as well as the contribution of the measured cross section to the leading-order hadronic vacuum polarization contribution to (g−2)μ(g−2)μ. We find this value to be aμππ,LO(600–900MeV)=(368.2±2.5stat±3.3sys)⋅10−10, which is between the corresponding values using the BaBar or KLOE data.