Measurement of $\Lambda$ transverse polarization in $e^{+}e^{-}$ collisions at $\sqrt{s}= 3.68-3.71$ GeV

TL;DR

This study measures the transverse polarization of Lambda baryons in electron-positron collisions at energies around 3.7 GeV, providing evidence for a non-zero phase between helicity states and precise form factor ratios.

Contribution

First measurement of Lambda transverse polarization at these energies, revealing a non-zero phase between helicity amplitudes and determining form factor ratios with statistical and systematic uncertainties.

Findings

Polarization significance at 2.6σ including systematics

Measured form factor ratio R^Ψ = 0.71 ± 0.10 (stat) ± 0.03 (sys)

Relative phase ΔΦ^Ψ = (23^{+8.8}_{-8.0} ± 1.6)°

Abstract

With data samples collected with the BESIII detector at seven energy points at $\sqrt{s}= 3.68 - 3.71$ GeV, corresponding to an integrated luminosity of 333 pb$^{-1}$, we present a study of the $\Lambda$ transverse polarization in the $e^+e^-\to\Lambda\bar\Lambda$ reaction. The significance of polarization by combining the seven energy points is found to be 2.6$\sigma$ including the systematic uncertainty, which implies a non-zero phase between the transition amplitudes of the $\Lambda\bar\Lambda$ helicity states. The modulus ratio and the relative phase of EM-$psionic$ form factors combined with all energy points are measured to be $R^{\Psi} =$ 0.71$^{+0.10}_{-0.10}$ $\pm$ 0.03 and $\Delta\Phi^{\Psi}$ = (23$^{+8.8}_{-8.0}$ $\pm$ 1.6$)^\circ$, where the first uncertainties are statistical and the second systematic.