We report a branching fraction measurement of the singly Cabibbo-suppressed decay Λ + c → ΛK + 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 Λ + c → ΛK + relative to that of the Cabibbo-favored decay= (4.78 ± 0.34 ± 0.20)%. Combining with the worldaverage value of B(Λ + c → Λπ + ), we obtain B(Λ + c → ΛK + ) = (6.21 ± 0.44 ± 0.26 ± 0.34) × 10 −4 . Here the first uncertainties are statistical, the second systematic, and the third comes from the uncertainty of the Λ + c → Λπ + 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.
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