We report a branching fraction measurement of the singly Cabibbo-suppressed decay ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}$ 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 $6.44{\mathrm{fb}}^{-1}$. The branching fraction of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}$ relative to that of the Cabibbo-favored decay ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+}$ is measured to be $\mathcal{R}=\frac{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+})}{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+})}=(4.78\pm 0.34\pm 0.20)\%$. Combining with the world-average value of $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+})$, we obtain $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+})=(6.21\pm 0.44\pm 0.26\pm 0.34)\times {10}^{-4}$. Here the first uncertainties are statistical, the second systematic, and the third comes from the uncertainty of the ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+}$ 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.

• Received 9 August 2022
• Accepted 16 November 2022

DOI:https://doi.org/10.1103/PhysRevD.106.L111101

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Published by the American Physical Society