KLOE measurement of [formula omitted] with Initial State Radiation and the [formula omitted] contribution to the muon anomaly

Abstract

The KLOE experiment at the ϕ – factory DAΦNE in Frascati near Rome is the first to have employed Initial State Radiation (ISR) to precisely determine the e + e − → π + π − ( γ ) cross section below 1 GeV. Such a measurement is particularly important to test the Standard Model (SM) calculation for the ( g − 2 ) of the muon, where a long standing 3 σ discrepancy is observed. In 2005 and 2008 KLOE published two measurements of the π + π − cross section with the photon emitted at small angles, and an independent measurement of the π + π − cross section with the photon emitted at large angles using data at a collision energy of 1 GeV (i.e., 20 MeV below the ϕ -peak) was published in 2010. While the measurements were normalized to the DAΦNE luminosity using large angle Bhabha scattering, a new analysis has been performed which derives the pion form factor directly from measuring the bin-by-bin π + π − γ / μ + μ − γ ratio. We present the preliminary results of this new measurement, as well as the previously published ones, and the impact on the evaluation of the hadronic contribution to the muon anomaly.