Measurement of the branching fraction of the singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}$

We report a branching fraction measurement of the singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}$ using a data sample collected with the BESIII detector at the BEPCII storage ring. The data span center-of-mass energies from 4.599 to 4.950 GeV and correspond to an integrated luminosity of $6.44\text{}\text{}{\mathrm{fb}}^{-1}$. The branching fraction of ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}$ relative to that of the Cabibbo-favored decay ${\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}$ is measured to be $R=\frac{B\left({\Lambda }_{c}^{+}\to \Lambda {K}^{+}\right)}{B\left({\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}\right)}=\left(4.78±0.34±0.20\right)%$. Combining with the world-average value of $B\left({\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}\right)$, we obtain $B\left({\Lambda }_{c}^{+}\to \Lambda {K}^{+}\right)=\left(6.21±0.44±0.26±0.34\right)×{10}^{-4}$. Here the first uncertainties are statistical, the second systematic, and the third comes from the uncertainty of the ${\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}$ branching fraction. This result, which is more precise than previous measurements, does not agree with theoretical predictions, and suggests that nonfactorizable contributions have been underestimated in current models.