Precise measurement of the e+e-→:π+π-(γ)cross section with the initial state radiation method at BAB

We present a precise BABAR measurement on the cross section of the process e+e-→π+π-(γ)from threshold to an energy of 3 GeV with the initial state radiation(ISR)technique,using 232 fb-1 of data collected with the BABAR detector at e+e-center-of-mass energies near 10.58 GeV.The ISR luminosity is determined from a study of the leptonic process e+e-→μ+μ-γ(γ).The leading-order hadronic contribution to the muon magnetic anomaly calculated using the ππ cross section measured from threshold to 1.8 GeV is(514.1±2.2(stat)±3.1(syst))×10-10.