First observation of Λ_c⁺→ΛK⁺π⁰ and evidence of Λ_c⁺→ΛK⁺π⁺π^-

We present the first observation of the singly Cabibbo-suppressed decay Λ_c⁺→ΛK⁺π⁰ with a significance of 5.7⁢𝜎 and the first evidence of Λ_c⁺→ΛK⁺π⁺π^- decay with a significance of 3.1⁢𝜎, 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  fb−1, in the center-of-mass energy range from 4.600 to 4.950 GeV. We determine the branching fractions ofΛ_c⁺→ΛK⁺π⁰ and Λ_c⁺→ΛK⁺π⁺π^- relative to their Cabibbo-favored counterparts to be ℬ⁡(Λ_c⁺→ΛK⁺π⁰)/ℬ⁡(Λ_c⁺→Λπ⁺π⁰)=(2.09±0.3⁢9stat±0.0⁢7syst)×10−2 and ℬ⁡(Λ_c⁺→ΛK⁺⁢π⁺π^-)/ℬ⁡(Λ_c⁺→Λπ⁺⁢π⁺π^-)=(1.13±0.4⁢1stat±0.0⁢6syst)×10−2, respectively. Moreover, by combining our measured result with the world average of ℬ⁡(Λ_c⁺→Λπ⁺π⁰), we obtain the branching fraction ℬ⁡(Λ_c⁺→ΛK⁺π⁰)=(1.49±0.2⁢7stat±0.0⁢5syst±0.0⁢8ref)×10−3. This result significantly departs from theoretical predictions based on quark 𝑆⁢𝑈⁡(3) flavor symmetry, which is underpinned by the presumption of meson pair 𝑆-wave amplitude dominance.