Observation of the Singly Cabibbo-Suppressed Decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{-}{K}^{+}{\pi }^{+}$

The singly Cabibbo-suppressed decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{-}{K}^{+}{\pi }^{+}$ is observed for the first time with a statistical significance of $6.4\sigma$ by using 4.5 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 ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{-}{K}^{+}{\pi }^{+}$ is measured to be $(3.8\pm {1.3}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}}\pm {0.2}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}})\times {10}^{-4}$ in a model-independent approach. This is the first observation of a Cabibbo-suppressed ${\mathrm{\Lambda }}_c^{+}$ decay involving ${\mathrm{\Sigma }}^{-}$ in the final state. The ratio of branching fractions between ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{-}{K}^{+}{\pi }^{+}$ and the Cabibbo-favored decay ${\mathrm{\Lambda }}_c^{+}\to {\mathrm{\Sigma }}^{-}{\pi }^{+}{\pi }^{+}$ is calculated to be $(0.4\pm 0.1)s_c^2$, where $s_c\equiv \sin {\theta }_c=0.2248$ with ${\theta }_c$ the Cabibbo mixing angle. This ratio significantly deviates from $1.0s_c^2$ and provides important information for the understanding of nonfactorization contributions in ${\mathrm{\Lambda }}_c^{+}$ decays.