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 φsymbol-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 < Mπ π< 0.975 GeV. For the latter we find Δπ πaμ= (387.2 ± 0.5stat± 2.4exp± 2.3th) × 10-10. © 2008 Elsevier B.V. All rights reserved.