First Measurement of the Decay Asymmetry in the pure W-boson-exchange Decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Xi }}^0{K}^{+}$

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 $\mathit{\text{W}}$-boson-exchange decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Xi }}^0{K}^{+}$ is studied with a full angular analysis. The corresponding decay asymmetry is measured for the first time to be ${\alpha }_{{\mathrm{\Xi }}^0{K}^{+}}=0.01\pm 0.16(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.03(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$. 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 ${\delta }_p-{\delta }_s=-1.55\pm 0.25(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.05(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\text{ rad}$ or $1.59\pm 0.25(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.05(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\text{ rad}$.