Observation of the singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to {\Sigma }^{-}{K}^{+}{\pi }^{+}$

The singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to {\Sigma }^{-}{K}^{+}{\pi }^{+}$ is observed for the first time with a statistical significance of $5.4\sigma $ by using $4.5\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ collision data collected at center-of-mass energies between 4.600 and 4.699 GeV with the BESIII detector at BEPCII. The absolute branching fraction of ${\Lambda }_{c}^{+}\to {\Sigma }^{-}{K}^{+}{\pi }^{+}$ is measured to be $\left(3.8±{1.2}_{\mathrm{stat}}±{0.2}_{\mathrm{syst}}\right)×{10}^{-4}$ in a model-independent approach. This is the first observation of a Cabibbo-suppressed ${\Lambda }_{c}^{+}$ decay involving ${\Sigma }^{-}$ in the final state. The ratio of branching fractions between ${\Lambda }_{c}^{+}\to {\Sigma }^{-}{K}^{+}{\pi }^{+}$ and the Cabibbo-favored decay ${\Lambda }_{c}^{+}\to {\Sigma }^{-}{\pi }^{+}{\pi }^{+}$ is observed to be $\left(0.4±0.1\right){s}_{c}^{2}$, where ${s}_{c}\equiv \mathrm{sin}{\theta }_{c}=0.2248$ with ${\theta }_{c}$ the Cabibbo mixing angle.