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 μ π π , 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.