Measurement of sigma(e+ e- -> pi+ pi-) from threshold to 0.85 GeV^2 using Initial State Radiation with the KLOE detector

We have measured the cross section of the radiative process e+e−→π+π−γe+e−→π+π−γ with the KLOE detector at the Frascati ϕ -factory DAΦNE, from events taken at a CM energy W=1 GeVW=1 GeV. Initial state radiation allows us to obtain the cross section for e+e−→π+π−e+e−→π+π−, the pion form factor |Fπ|2|Fπ|2 and the dipion contribution to the muon magnetic moment anomaly, View the MathML sourceΔaμππ=(478.5±2.0stat±5.0syst±4.5th)×10−10 in the range View the MathML source0.1<Mππ2<0.85 GeV2, where the theoretical error includes a SU(3) χPTχPT estimate of the uncertainty on photon radiation from the final pions. The discrepancy between the Standard Model evaluation of aμaμ and the value measured by the Muon g-2 collaboration at BNL is confirmed.