Measurement of the proton form factor at very low Q2

Abstract

After the precise determination of the proton radius via the spectroscopy of muonic hydrogen, the proton radius puzzle arose in 2010. The extracted proton radius differs for over $7$ standard deviations from the results obtained from electron scattering experiments and hydrogen spectroscopy. To help understanding this discrepancy an experiment has been performed at the Mainz Microtron (MAMI) in order to measure the proton form factors at the lowest possible values of Q$^2$ to date, using a method based on initial state radiation (ISR). In the following the problem will be briefly discussed and then the ISR approach will be described. Then the analysis will be presented and preliminary results will be shown. At the end an outlook for further exploiting the ISR method will be given.