Time-like nucleon form factor measurements at $\overline{\textbf{P}}\textbf{ANDA}$

Abstract

The electromagnetic probe is an excellent tool to investigate the structure of the nucleon. The nearly 4π detector PANDA, to be installed on the FAIR accelerator complex at Darmstadt, will allow to make a precise determination of the electromagnetic form factors of the proton in the time-like (TL) region with unprecedented precision. In the framework of the one-photon exchange, the center of mass unpolarized differential cross section of the reaction $\overline{p} p \rightarrow e^+ e^-$ is a linear combination of the squared moduli of the electric $G^{p}_E$ and magnetic $G^{p}_M$ proton form factors. The precise measurement of the angular distribution over almost full angular range then directly gives these quantities. Only two experiments have provided the ratio $R = {|G^{p}_E|/|G^{p}_M|}$ but with very large statistical uncertainties. Within PANDA, there is a unique opportunity to measure separately the moduli of these two proton form factors $G^{p}_E$ and $G^{p}_M$ in good conditions, up to around q ² = 14 GeV² .