Precision measurement of σ (e+e- →π+π- γ) / σ (e+e- →μ+μ- γ) and determination of the π+π- contribution to the muon anomaly with the KLOE detector

We have measured the ratio σ (e⁺e^- →π⁺π^- γ) / σ (e⁺e^- →μ⁺μ^- γ), with the KLOE detector at DAΦNE for a total integrated luminosity of ∼ 240pb^-1. From this ratio we obtain the cross section σ (e⁺e^- →π⁺π^-). From the cross section we determine the pion form factor ^{|F_π | 2} and the two-pion contribution to the muon anomaly a_μ for 0.592 <M_ππ < 0.975 GeV, Δ^ππa_μ = (385.1 ±1.1_stat ±2.7_sys+theo) ×10^-10. This result confirms the current discrepancy between the Standard Model calculation and the experimental measurement of the muon anomaly.