Observation of $ {\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{a}_{0}(980{)}^{+}$ and Evidence for $ \mathrm{\Sigma }(1380{)}^{+}$ in $ {\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{\pi }^{+}\eta $

Based on $ 6.1\text{}\text{}{\mathrm{fb}}^{-1}$ of $ {e}^{+}{e}^{-}$ annihilation data collected at center-of-mass energies from 4.600 to 4.843 GeV with the BESIII detector at the BEPCII collider, a partial wave analysis of $ {\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{\pi }^{+}\eta $ is performed, and branching fractions and decay asymmetry parameters of intermediate processes are determined. The process $ {\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{a}_{0}(980{)}^{+}$ is observed for the first time, and evidence for the pentaquark candidate $ \mathrm{\Sigma }(1380{)}^{+}$ decaying into $ \mathrm{\Lambda }{\pi }^{+}$ is found with statistical significance larger than $ 3\sigma $ with mass and width fixed to theoretical predictions. The branching fraction product $ \mathcal{B}[{\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{a}_{0}(980{)}^{+}]\mathcal{B}[{a}_{0}(980{)}^{+}\to {\pi }^{+}\eta ]$ is determined to be $ (1.05\pm 0.1{6}_{\mathrm{stat}}\pm 0.0{5}_{\mathrm{syst}}\pm 0.0{7}_{\mathrm{ext}})\%$, which is larger than theoretical calculations by 1–2 orders of magnitude. Here the third (external) systematic is from $ \mathcal{B}({\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{\pi }^{+}\eta )$. Finally, we precisely obtain the absolute branching fraction $ \mathcal{B}({\mathrm{\Lambda }}_{c}^{+}\to \mathrm{\Lambda }{\pi }^{+}\eta )=(1.94\pm \phantom{\rule{0ex}{0ex}}0.0{7}_{\mathrm{stat}}\pm 0.1{1}_{\mathrm{syst}})\%$.