The Production and Decay Dynamics of the Charmed Baryon $\Lambda_c^+$ in $e^+e^-$ Annihilations near Threshold

TL;DR

This paper reports the first observation of transverse polarization of the $ ext{Lambda}_c^+$ baryon in electron-positron annihilations near threshold, providing new insights into its production dynamics and potential $C ext{P}$ violation effects.

Contribution

It presents the first measurement of $ ext{Lambda}_c^+$ transverse polarization and decay asymmetries in $e^+e^-$ collisions near threshold, revealing new aspects of charmed baryon production and decay.

Findings

First observation of $ ext{Lambda}_c^+$ transverse polarization.

Extraction of decay asymmetry parameters and phase shifts.

Enhanced sensitivity to $C ext{P}$ violation in charmed baryons.

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 non-zero 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 $\Lambda_c^+ \to pK_S^0$, $\Lambda\pi^+$, $\Sigma^0\pi^+$, and $\Sigma^+\pi^0$ are simultaneously extracted from the joint angular distributions. From these parameters, both the weak and strong phase shifts are extracted, and several $C\!P$ observables are tested. The obtained results show that the transverse polarization of the produced baryon significantly enhances the capability of searching for $C\!P$-violating effects, opening a novel way for exploring $C\!P$ violation in the charmed baryon sector.