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

Abstract We have measured the ratio σ ( e + e − → π + π − γ ) / σ ( e + e − → μ + μ − γ ) , with the KLOE detector at DA Φ NE for a total integrated luminosity of ∼ 240 pb − 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 π π 0.975 GeV , Δ π π a 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.