Nucleon polarization in the process $e^+e^-\to N\bar{N}$ near threshold

TL;DR

This paper investigates nucleon polarization in electron-positron annihilation near threshold, showing how polarization measurements can reveal electromagnetic form factors and their phase, with implications for understanding nucleon structure.

Contribution

It introduces a method to determine nucleon electromagnetic form factors and their phase from polarization measurements near threshold using the Paris optical potential.

Findings

Nucleon polarization strongly depends on energy near threshold.

Polarization degree is high enough for experimental detection.

Polarization is significant even with unpolarized beams.

Abstract

The process $e^+e^- \rightarrow N\bar N$ is studied nearby a threshold with account for polarizations of all initial and final particles. The nucleon polarization $\bm \zeta^N$ reveals a strong energy dependence due to that of the nucleon electromagnetic form factors $G_E(Q^2)$ and $G_M(Q^2)$ caused by the final-state interaction of nucleons. It is shown that the modulus of the ratio of these form factors and their relative phase can be determined by measuring $\bm \zeta^N$ along with the differential cross section. The polarization degree is analyzed using Paris $N\bar N$ optical potential for calculation of the form factors. It turns out that $|\bm \zeta^N|$ is high enough in a rather wide energy range above the threshold. Being especially high for longitudinally polarized beams, $|\bm \zeta^N|$ is noticeable even if both $e^+e^-$ beams are unpolarized.