First observation of the decay ${\Lambda }_{c}^{+}\to n{K}_{S}^{0}{\pi }^{+}{\pi }^{0}$

Based on $4.5\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ collision data accumulated at center-of-mass energies between 4599.53 and 4698.82 MeV with the BESIII detector, the decay ${\Lambda }_{c}^{+}\to n{K}_{S}^{0}{\pi }^{+}{\pi }^{0}$ is observed for the first time with a significance of $9.2\sigma $. The branching fraction is measured to be $\left(0.85±0.13±0.03\right)%$, where the first uncertainty is statistical and the second is systematic, which differs from the theoretical prediction based on isospin by $4.4\sigma $. This indicates that there may be resonant contributions or some unknown dynamics in this decay.