KLOE Note no 225 June 2, 2011 Measurement of σ(e +e− → π π−) for M2 ππ between 0.1 and 0.85 GeV2 using the Initial State Radiation method with the KLOE detector Paolo B
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KLOE Note n o 225 June 2, 2011 Measurement of σ (e + e − → π + π − ) for M 2 ππ between 0.1 and 0.85 GeV 2 using the Initial State Radiation method with the KLOE detector Paolo Beltrame a,d , Achim Denig a , Stefan E. M¨ uller a,e , Wolfgang Kluge b , Federico Nguyen c a Institut f¨ ur Kernphysik, Universit¨ at Mainz, Germany b Institut f¨ ur Experimentelle Kernphysik, Universit¨ at Karlsruhe, Germany c Dipartimento di Fisica dell’Universit` a “Roma Tre” e Sezione INFN, Rome, Italy d now at: UCLA Physics and Astronomy Dept., Los Angeles, California 90095-1547, USA e now at: KVI, Rijksuniversiteit Groningen, The Netherlands Abstract The KLOE experiment at the φ-factory DAΦNE has measured the pion form factor in the range 0.1 <M 2 ππ < 0.85 GeV 2 for the squared invariant mass of the two-pion system M 2 ππ using events taken at √ s = 1 GeV with a photon emitted in the initial state at large polar angle. This measurement extends the M 2 ππ region covered by previous KLOE ISR measurements down to the two-pion production threshold. It allows to compute the dipion contribution Δa ππ μ to the anomaly of the muon magnetic moment a μ =(g μ − 2)/2. Our value of Δa ππ μ = (478.5 ± 2.0 stat ± 5.0 syst ± 4.5 theo ) × 10 −10 further confirms the discrepancy between the Standard Model evaluation for a μ and the experimental value measured by the (g μ − 2)-collaboration at BNL. 1
