First observation of the decay ${\Lambda }_c^{+}\to n{K}_S^0{\pi }^{+}{\pi }^0$

Based on $4.5{\mathrm{fb}}^{-1}$ 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 $(0.85\pm 0.13\pm 0.03)\%$, 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.