The Radiative Return: a review of experimental results

The radiative return is a new method for hadronic cross section measurements at electron-positron colliders, which are operated at a fixed center-of-mass energy (so-called particle factories). In order to lower the effective hadronic mass Mhadr only such events are taken, in which one of the electrons or positrons has emitted an initial state radiation photon. We present precision measurements of the pion formfactor from the Frascati ϕ-factory DAΦNE with the KLOE experiment and measurements of higher particle multiplicities as well as a measurement of the timelike proton-antiproton formfactor from the BaBar experiment at the B-factory PEP-II. These raditative return measurements are compared to results, which are obtained by means of an energy scan, i.e. by means of a systematic variation of the beam energy of the collider. We also report on the impact of these measurements on the hadronic contribution of the anomalous magnetic moment of the muon, which is obtained via a dispersion integral using hadronic cross section data as input.