First Measurement of the Decay Asymmetry in the Pure $W$-Boson-Exchange Decay ${\Lambda }_c^{+}\to {\Xi }^0{K}^{+}$

Based on $4.4{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at the center-of-mass energies between 4.60 and 4.70 GeV with the BESIII detector at the BEPCII collider, the pure $W$-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 ${\alpha }_{{\Xi }^0{K}^{+}}=0.01\pm 0.16\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)$. This result reflects the noninterference effect between the $S$- and $P$-wave amplitudes. The phase shift between $S$- and $P$-wave amplitudes has two solutions, which are ${\delta }_p-{\delta }_s=-1.55\pm 0.25\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)\mathrm{rad}$ or $1.59\pm 0.25\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)\mathrm{rad}$.