Measurement of the $mathrm e^+mathrm e^-rightarrowmathrmpi^+mathrmpi^-$ Cross Section between 600 and 900 MeV Using Initial State Radiation

We extract the $e^+e^-rightarrow pi^+pi^-$ 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 $|F_pi|^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2)_mu$. We find this value to be $a_mu^{pipi,rm LO}(600-900;rm MeV) = (368.2 pm 2.5_{rm stat} pm 3.3_{rm sys})cdot 10^{-10}$.