MEASUREMENT OF THE HADRONIC CROSS SECTION AT KLOE/KLOE-2

Abstract

The KLOE experiment at the DAΦNE ϕ-factory in Frascati was the first to use Initial State Radiation to precisely determine the e+e− → π+π−(γ) cross section below 1 GeV. This measurement is particularly important to test the Standard Model calculation for the anomalous magnetic moment of the muon, (g-2), where a long standing 3σ discrepancy between measurement and theory is observed. In 2005 and 2008 KLOE published two measurements of the π+π− cross section with the photon emitted at small angle. In 2011 an independent measurement of the π+π− cross section with the photon emitted at large angle was published using data at a collision energy of 1 GeV, 20 MeV below the ϕ-peak. A new analysis, published in 2013, derives the pion form factor directly from the bin-by-bin π+π−γ/μ+μ−γ ratio significantly reducing the systematic uncertainty, while the previous measurements were normalized to the DAΦNE luminosity using large angle Bhabha scattering. We present the final result of this measurement, as well as a preliminary value for the combined result of all KLOE hadronic cross section measurements, and evaluate the hadronic contribution to the muon anomaly.