Measurement of σ(e+e−→π+π−) from threshold to 0.85 GeV2 using initial state radiation with the KLOE detector

Abstract We have measured the cross section of the radiative process e + e − → π + π − γ with the KLOE detector at the Frascati ϕ -factory DAΦNE, from events taken at a CM energy W = 1 GeV . Initial state radiation allows us to obtain the cross section for e + e − → π + π − , the pion form factor | F π | 2 and the dipion contribution to the muon magnetic moment anomaly, Δ a μ π π = ( 478.5 ± 2.0 stat ± 5.0 syst ± 4.5 th ) × 10 − 10 in the range 0.1 M π π 2 0.85 GeV 2 , where the theoretical error includes a SU(3) χ PT estimate of the uncertainty on photon radiation from the final pions. The discrepancy between the Standard Model evaluation of a μ and the value measured by the Muon g-2 collaboration at BNL is confirmed.