First Measurement of the Decay Asymmetry in the pure W-boson-exchange Decay $Λ_{c}^{+}\toΞ^{0}K^{+}$

Abstract: Based on $4.4~\text{fb}^{-1}$ of $e^{+}e^{-}$ annihilation data collected at the center-of-mass energies between $4.60$ and $4.70~\text{GeV}$ with the BESIII detector at the BEPCII collider, the pure \textit{W}-boson-exchange decay $\Lambda_{c}^{+}\to\Xi^{0}K^{+}$ is studied with a full angular analysis. The corresponding decay asymmetry is measured for the first time to be $\alpha_{\Xi^{0}K^{+}}=0.01\pm0.16({\rm stat.})\pm0.03({\rm syst.})$. This result reflects the non-interference effect between the $S$- and $P$-wave amplitudes. The phase shift between $S$- and $P$-wave amplitudes has two solutions, which are $\delta_{p}-\delta_{s}=-1.55\pm0.25({\rm stat.})\pm0.05({\rm syst.})~\text{rad}$ or $1.59\pm0.25({\rm stat.})\pm0.05({\rm syst.})~\text{rad}$.