First observation of Λc+ → Λk+π0 and evidence of Λc+ → Λk+π+π-

We present the first observation of the singly Cabibbo-suppressed decay Λc+→ΛK+π0 with a significance of 5.7σ and the first evidence of Λc+→ΛK+π+π- decay with a significance of 3.1σ, 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 of Λc+→ΛK+π0 and Λc+→ΛK+π+π- relative to their Cabibbo-favored counterparts to be B(Λc+→ΛK+π0)B(Λc+→Λπ+π0)=(2.09±0.39stat±0.07syst)×10-2 and B(Λc+→ΛK+π+π-)B(Λc+→Λπ+π+π-)=(1.13±0.41stat±0.06syst)×10-2, respectively. Moreover, by combining our measured result with the world average of B(Λc+→Λπ+π0), we obtain the branching fraction B(Λc+→ΛK+π0)=(1.49±0.27stat±0.05syst±0.08ref)×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.