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

We present the first observation of the singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{0}$ with a significance of $5.7\sigma $ and the first evidence of ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{+}{\pi }^{-}$ 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 $6.4\text{}\text{}{\mathrm{fb}}^{-1}$, in the center-of-mass energy range from 4.600 to 4.950 GeV. We determine the branching fractions of ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{0}$ and ${\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{+}{\pi }^{-}$ relative to their Cabibbo-favored counterparts to be $\frac{B\left({\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{0}\right)}{B\left({\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}{\pi }^{0}\right)}=\left(2.09±0.3{9}_{\mathrm{stat}}±0.0{7}_{\mathrm{syst}}\right)×{10}^{-2}$ and $\frac{B\left({\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{+}{\pi }^{-}\right)}{B\left({\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}{\pi }^{+}{\pi }^{-}\right)}=\phantom{\rule{0ex}{0ex}}\left(1.13±0.4{1}_{\mathrm{stat}}±0.0{6}_{\mathrm{syst}}\right)×{10}^{-2}$, respectively. Moreover, by combining our measured result with the world average of $B\left({\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}{\pi }^{0}\right)$, we obtain the branching fraction $B\left({\Lambda }_{c}^{+}\to \Lambda {K}^{+}{\pi }^{0}\right)=\phantom{\rule{0ex}{0ex}}\left(1.49±0.2{7}_{\mathrm{stat}}±0.0{5}_{\mathrm{syst}}±0.0{8}_{\mathrm{ref}}\right)×{10}^{-3}$. This result significantly departs from theoretical predictions based on quark $SU\left(3\right)$ flavor symmetry, which is underpinned by the presumption of meson pair $S$-wave amplitude dominance.