Observation of the Singly Cabibbo Suppressed Decay ${\Lambda }_{c}^{+}\to n{\pi }^{+}$

The singly Cabibbo-suppressed decay ${\Lambda }_{c}^{+}\to n{\pi }^{+}$ is observed for the first time with a statistical significance of $7.3\sigma $ by using $3.9\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ collision data collected at center-of-mass energies between 4.612 and 4.699 GeV with the BESIII detector at BEPCII. The branching fraction of ${\Lambda }_{c}^{+}\to n{\pi }^{+}$ is measured to be $\left(6.6±1.{2}_{\mathrm{stat}}±0.{4}_{\mathrm{syst}}\right)×{10}^{-4}$. By taking the upper limit of branching fractions of ${\Lambda }_{c}^{+}\to p{\pi }^{0}$ from the Belle experiment, the ratio of branching fractions between ${\Lambda }_{c}^{+}\to n{\pi }^{+}$ and ${\Lambda }_{c}^{+}\to p{\pi }^{0}$ is calculated to be larger than 7.2 at the 90% confidence level, which disagrees with most predictions of the available phenomenological models. In addition, the branching fractions of the Cabibbo-favored decays ${\Lambda }_{c}^{+}\to \Lambda {\pi }^{+}$ and ${\Lambda }_{c}^{+}\to {\Sigma }^{0}{\pi }^{+}$ are measured to be $\left(1.31±0.0{8}_{\mathrm{stat}}±0.0{5}_{\mathrm{syst}}\right)×{10}^{-2}$ and $\left(1.22±0.0{8}_{\mathrm{stat}}±0.0{7}_{\mathrm{syst}}\right)×{10}^{-2}$, respectively, which are consistent with previous results.