Measurement of $\Lambda$ baryon polarization in $e^+e^-\rightarrow\Lambda\bar\Lambda$ at $\sqrt{s} = 3.773$ GeV

TL;DR

This study measures the polarization of Lambda baryons produced in electron-positron collisions at 3.773 GeV, providing insights into the phase and ratio of their electromagnetic form factors, with results indicating a small but notable polarization.

Contribution

First measurement of Lambda baryon polarization at this energy, determining the phase difference and form factor ratio with statistical and systematic uncertainties.

Findings

Polarization significance is 2σ, suggesting a zero phase within uncertainties.

Phase difference between helicity states is approximately 71 degrees with large uncertainties.

Form factor ratio is about 0.48, indicating the relative strength of electric and magnetic form factors.

Abstract

Using a data sample of $\psi(3770)$ events collected with the BESIII detector at BEPCII corresponding to an integrated luminosity of 2.9 fb$^{-1}$, we report a measurement of $\Lambda$ spin polarization in $e^+e^-\rightarrow\Lambda\bar\Lambda$ at $\sqrt{s} = 3.773$ GeV. The significance of polarization is found to be 2$\sigma$ including the systematic uncertainty, which implies a zero phase between the transition amplitudes of the $\Lambda\bar{\Lambda}$ helicity states. This phase can be interpreted in terms of psionic form factors, and is determined to be $\Delta\Phi^{\Psi}$ = $\Phi^{\Psi}_{E} - \Phi^{\Psi}_{M}$ = $(71^{+66}_{-46}$ $\pm$ 5)$^{\circ}$. Similarly, the ratio between the form factors is found to be $R^{\psi}$ = $|G^{\Psi}_{E}/G^{\Psi}_{M}|$ = $0.48^{+0.12}_{-0.07}$ $\pm$ 0.04. The first uncertainties are statistical and the second systematic.