Experimental study of the $e^+e^-\to n\bar n$ process at the VEPP-2000 $e^+e^-$ collider with the SND detector

TL;DR

This study measures the cross section and form factors of the neutron-antineutron production process in electron-positron collisions at energies near 2 GeV, providing new insights into neutron electromagnetic structure.

Contribution

The paper presents the first measurement of the $e^+e^- o nar n$ cross section and neutron form factors at VEPP-2000, using a novel time measurement technique in the calorimeter.

Findings

Cross section varies from 0.5 to 0.35 nb between 1.894 and 2 GeV.

Effective neutron form factor is derived and compared with proton form factor.

Neutron electric and magnetic form factors ratio is consistent with unity.

Abstract

The process $e^+e^-\to n\bar n$ is studied in the experiment at the VEPP-2000 $e^+e^-$ collider with the SND detector. The technique of the time measurements in the multichannel NaI(Tl) electromagnetic calorimeter is used to select nnbar events. The value of the measured cross section in the center-of-mass energy range from 1.894 to 2 GeV varies from 0.5 to 0.35 nb. The effective neutron timelike form factor is derived from the measured cross section and compared with the proton form factor. The ratio of the neutron electric and magnetic form factors is obtained from the analysis of the antineutron polar angle distribution and found to be consistent with unity.