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 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 $\mathcal{ R}=\frac{ \mathcal{ B}(\Lambda_{ c}^{ +}\to\Lambda K^{ +})}{ \mathcal{ B}(\Lambda_{ c}^{ +}\to\Lambda \pi^{ +})}=(4.78\pm0.34\pm0.20)\%$. Combining with the world-average value of $\mathcal{ B}(\Lambda_{ c}^{ +}\to\Lambda \pi^{ +})$, we obtain $\mathcal{ B}(\Lambda_c^+\to\Lambda K^+)=(6.21\pm0.44\pm0.26\pm0.34)\times 10^{ -4}$. 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 non-factorizable contributions have been under-estimated in current models.
Categories: hep-ex