Measurement of σ (e + e − → π + π − γ (γ )) and the dipion contribution to the muon anomaly with the KLOE detector

We have measured the cross section σ(e + e − →π + π − γ(γ)) σ ( e + e − → π + π − γ ( γ ) ) at DAΦNE, the Frascati ϕ -factory, using events with initial state radiation photons emitted at small angle and inclusive of final state radiation. We present the analysis of a new data set corresponding to an integrated luminosity of 240 pb −1 . We have achieved a reduced systematic uncertainty with respect to previously published KLOE results. From the cross section we obtain the pion form factor and the contribution to the muon magnetic anomaly from two-pion states in the mass range 0.592 ππ 0.592 M π π 0.975 GeV . For the latter we find Δππa μ =(387.2±0.5 stat ±2.4 exp ±2.3 th )× 10 −10 Δ π π a μ = ( 387.2 ± 0.5 stat ± 2.4 exp ± 2.3 th ) × 10 −10 .