Production and Decay Dynamics of the Charmed Baryon ${\Lambda }_c^{+}$ in $e^{+}{e}^{-}$ Annihilations near Threshold

Abstract

The study of the charmed baryons is crucial for investigating the strong and weak interactions in the standard model and for gaining insights into the internal structure of baryons. In an $e^{+}{e}^{-}$ experiment the lightest charmed baryon, ${\Lambda }_c^{+}$, can be produced in pairs through the single-photon annihilation process. This process can be described by two complex electromagnetic form factors. The presence of a nonzero relative phase between these form factors gives rise to a transverse polarization of the charmed baryon and provides additional constraints on the dynamic parameters in the decays. In this article, we present the first observation of the transverse polarization of ${\Lambda }_c^{+}$ in the reaction $e^{+}{e}^{-}\to {\Lambda }_c^{+}{\bar{\Lambda }}_c^{-}$, based on $6.4{\text{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at center-of-mass energies between $4600$~MeV and $4951$~MeV with the BESIII detector. The decay asymmetry parameters in the decays ${\Lambda }_c^{+}\to p{K}_S^0$, $\Lambda {\pi }^{+}$, ${\Sigma }^0{\pi }^{+}$, and ${\Sigma }^{+}{\pi }^0$ are simultaneously extracted from the joint angular distributions. By independently extracting the above parameters for the process and its $CP$-conjugate counterpart, we derive both the strong and weak phase differences between different partial waves, and test several $CP$ observables. The obtained results fully demonstrate the feasibility of exploiting transverse polarization at electron-positron colliders to search for $CP$-violating effects, opening up a new avenue for exploring $CP$ violation in the charmed baryon sector.

20 pages,9 figures,4 tables; Accepted by Physical Review X