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 ) ) 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 M π π 0.975 GeV . For the latter we find Δ π π a μ = ( 387.2 ± 0.5 stat ± 2.4 exp ± 2.3 th ) × 10 −10 .