Observation of Transverse Polarization and Determination of Psionic Form Factors of the $\Lambda$ Hyperon at $\sqrt{s}=3.773$ GeV

Abstract

Using a data sample of $e^{+}{e}^{-}$ collision data corresponding to an integrated luminosity of 20.3 fb${}^{-1}$ collected by the BESIII detector at the BEPCII collider, we present an observation of transverse polarization and a complete determination of the psionic form factors of the $\Lambda$ hyperon in $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$ decay with the entangled $\Lambda -\bar{\Lambda }$ pair at $\sqrt{s}=3.773$ GeV. The relative phase between the psionic form factors is determined to be $\Delta \Phi =(1.53\pm 0.36\pm 0.03)$~rad with a significance of 5.5 $\sigma$ taking into account systematic uncertainty. This result indicates a non-zero phase between the transition amplitudes of the $\Lambda \bar{\Lambda }$ helicity states. Additionally, we measure the angular distribution parameter and the modulus of the ratio between the psionic form factors to be $\eta =0.86\pm 0.05\pm 0.03$ and $R(s)=|G_E(s)/G_M(s)|=0.47\pm 0.08\pm 0.05$, where the first uncertainty is statistical and the second systematic.

arXiv comment9 pages, 5 figures, consistent with the publication in Phys. Rev. D (Letter) 112, L051102 (2025)

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• Observation of $\Xi (1530{)}^0$ Polarization and Determination of Its Electric and Magnetic Moments in $\psi (3686)\to \Xi (1530{)}^0\bar{\Xi }(1530{)}^0$