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

Based on $4.4{\mathrm{fb}}^{-1}$ 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 ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Xi }}^0{K}^{+}$ is studied with a full angular analysis. The corresponding decay asymmetry is measured for the first time to be ${\alpha }_{{\mathrm{\Xi }}^0{K}^{+}}=0.01\pm 0.16(\mathrm{stat})\pm 0.03(\mathrm{syst})$. 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 ${\delta }_p-{\delta }_s=-1.55\pm 0.25(\mathrm{stat})\pm 0.05(\mathrm{syst})\mathrm{rad}$ or $1.59\pm 0.25(\mathrm{stat})\pm 0.05(\mathrm{syst})\mathrm{rad}$.

Figure 1

Feynman diagrams for ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Xi }}^0{K}^{+}$.

Figure 2

Distribution of the $M_{\mathrm{BC}}$ fitting result at 4.60 GeV, and the corresponding signal yield is $70\pm 8$. Black points with error bars are data; the blue shaded region indicates the combinatorial background events and the pink shaded region is the misreconstructed signal events.

Figure 3

Definitions of the helicity frames and related angles for $e^{+}{e}^{-}\to {\mathrm{\Lambda }}_c^{+}{\overline{\mathrm{\Lambda }}}_c^{-}$, ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Xi }}^0{K}^{+}$, ${\mathrm{\Xi }}^0\to \mathrm{\Lambda }{\pi }^0$, and $\mathrm{\Lambda }\to p{\pi }^{-}$.

Figure 4

Projections of the best fit onto various variables. Black points with error bars are data; red solid lines are phase space MC events reweighted by angular distribution formula, and represent the fitting result; the blue shaded region denotes the combinatorial background events and the pink shaded region is the misreconstructed signal events.

Figure 5

Comparison between this Letter and theoretical predictions, where the branching fraction is taken from PDG (2022) [2]. The $1\sigma$, $2\sigma$, and $3\sigma$ contours correspond to 68.2%, 95.4%, and 99.7% conference level, respectively. The blue symbols are theoretical predictions and the red star is the result from this Letter. The definitions of the superscripts $a$ and $b$ can be found in Table 1 and the theory acronyms are explained in the text.