First measurement of proton's charge form factor at very low Q2 with initial state radiation

Abstract

We report on a new experimental method based on initial-state radiation (ISR) in e–p scattering, which exploits the radiative tail of the elastic peak to study the properties of electromagnetic processes and to extract the proton charge form factor (GEp) at extremely small Q2. The ISR technique was implemented in an experiment at the three-spectrometer facility of the Mainz Microtron (MAMI). This led to a precise validation of radiative corrections far away from elastic line and provided first measurements of GEp for 0.001≤Q2≤0.004(GeV/c)2.