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 $\mathrm{ fb}^{ -1}$ 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 $(6.6\pm1.2_{ \rm stat}\pm0.4_{ \rm syst})\times 10^{ -4}$. 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 the current predictions of 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 $(1.31\pm0.08_{ \rm stat}\pm0.05_{ \rm syst})\times 10^{ -2}$ and $(1.22\pm0.08_{ \rm stat}\pm0.07_{ \rm syst})\times 10^{ -2}$, respectively, which are consistent with previous results.
Categories: hep-ex