Measurement of σ ( e + e ? → π + π ? ) from threshold to 0.85 GeV 2 using initial state radiation with the KLOE detector

摘要

We have measured the cross section of the radiative process e + e ? → π + π ? γ with the KLOE detector atthe Frascati φ -factory DA ? NE, from events taken at a CM energy W = 1 GeV. Initial state radiation allowsus to obtain the cross section for e + e ? → π + π ? , the pion form factor | F π | 2 and the dipion contributionto the muon magnetic moment anomaly, ? a ππμ= ( 478 . 5 ± 2 . 0 stat ± 5 . 0 syst ± 4 . 5 th ) × 10 ? 10 in the range0 . 1 < M 2ππ < 0 . 85 GeV2 , where the theoretical error includes a SU(3) χ PT estimate of the uncertainty onphoton radiation from the final pions. The discrepancy between the Standard Model evaluation of a μ andthe value measured by the Muon g-2 collaboration at BNL is confirmed.