# Complete measurement of $\Lambda$ electromagnetic form factors

## TL;DR

This paper presents the first complete measurement of the electromagnetic form factors of the Lambda baryon in the time-like region, including the ratio, phase, and cross section, using data from the BESIII detector at 2.396 GeV.

## Contribution

It provides the first measurement of the relative phase between electric and magnetic form factors and a complete determination of Lambda baryon electromagnetic form factors.

## Key findings

- Measured the ratio |G_E/G_M| as 0.96±0.14 (stat.)±0.02 (sys.)
- Determined the phase difference ΔΦ as 37°±12° (stat.)±6° (sys.)
- Measured the cross section as 118.7±5.3 (stat.)±5.1 (sys.) pb

## Abstract

The exclusive process $e^+e^-\rightarrow\Lambda\bar{\Lambda}$, with $\Lambda \to p\pi^-$ and $\bar{\Lambda} \to \bar{p}\pi^+$, has been studied at $\sqrt{s} =$ 2.396 GeV for measurement of the $\Lambda$ electric and magnetic form factors, $G_E$ and $G_M$. A data sample, corresponding to an integrated luminosity of 66.9 pb$^{-1}$, was collected with the BESIII detector for this purpose. A multi-dimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio $R=|G_E/G_M|$ and, for the first time for any baryon, the relative phase $\Delta\Phi=\Phi_E-\Phi_M$. The resulting values are obtained using the recent and most precise measured value of the asymmetry parameter $\alpha_{\Lambda}$ = $0.750~\pm~0.010$ to be $R~=~0.96\pm0.14~(\rm stat.)\pm~0.02~(\rm sys.)$ and $\Delta\Phi=37^{\mathrm{o}}\pm~12^{\mathrm{o}}~(\rm stat.)\pm~6^{\mathrm{o}}~(\rm sys.)$, respectively. In addition, the cross section is measured with unprecedented precision to be $\sigma = 118.7~\pm~5.3~(\rm stat.)\pm~5.1~(\rm sys.)$ pb, which corresponds to an effective form factor of $|G|=0.123~\pm~0.003~(\rm stat.)\pm~0.003~(\rm sys.)$. The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon time-like electromagnetic form factors.