First observation of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^0$ and evidence of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^{+}{\pi }^{-}$

We present the first observation of the singly Cabibbo-suppressed decay ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^0$ with a significance of $5.7\sigma$ and the first evidence of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^{+}{\pi }^{-}$ decay with a significance of $3.1\sigma$, based on $e^{+}{e}^{-}$ annihilation data recorded by the BESIII detector at the BEPCII collider. The data correspond to an integrated luminosity of $6.4\mathrm{f}{\mathrm{b}}^{-1}$, in the center-of-mass energy range from $4.600\mathrm{G}\mathrm{e}\mathrm{V}$ to $4.950\mathrm{G}\mathrm{e}\mathrm{V}$. We determine the branching fractions of ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^0$ and ${\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^{+}{\pi }^{-}$ relative to their Cabibbo-favored counterparts to be $\frac{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^0)}{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+}{\pi }^0)}=(2.09\pm {0.39}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.07}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-2}$ and $\frac{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^{+}{\pi }^{-})}{\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+}{\pi }^{+}{\pi }^{-})}=(1.13\pm {0.41}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.06}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}})\times {10}^{-2}$, respectively. Moreover, by combining our measured result with the world average of $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{\pi }^{+}{\pi }^0)$, we obtain the branching fraction $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to \mathrm{\Lambda }{K}^{+}{\pi }^0)=(1.49\pm {0.27}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.}}\pm {0.05}_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}}\pm {0.08}_{\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{.}})\times {10}^{-3}$. This result significantly departs from theoretical predictions based on quark $SU(3)$ flavor symmetry, which is underpinned by the presumption of meson pair $S$-wave amplitude dominance.