Observation of Λ+c → nπ+η and search for Λ+c → na0(980)+

By analysing 6.1 fb − 1 of data collected at centre-of-mass energies between $\sqrt{s}=4.600$ and 4.843 GeV with the BESIII detector at the BEPCII collider, we observe the decay ${\Lambda}_c^{+}\to n{\pi}^{+}\eta$ for the first time with a statistical significance of 9.5 σ . The ratio of branching fractions $\mathcal{B}\left({\Lambda}_c^{+}\to n{\pi}^{+}\eta \right)/\mathcal{B}\left({\Lambda}_c^{+}\to \Lambda {\pi}^{+}\eta \right)$ is measured to be 0.155 ± 0.031 stat. ± 0.012 syst. Taking the world average of $\mathcal{B}\left({\Lambda}_c^{+}\to \Lambda {\pi}^{+}\eta \right)$ as reference, the absolute branching fraction is calculated to be $\mathcal{B}\left({\Lambda}_c^{+}\to n{\pi}^{+}\eta \right)=\left(2.94\pm {0.59}_{\mathrm{stat}.}\pm {0.23}_{\mathrm{syst}.}\pm {0.13}_{\mathrm{ref}.}\right)\times {10}^{-3}$ . The intermediate process ${\Lambda}_c^{+}\to n{a}_0{(980)}^{+}$ is also searched for in the π + η invariant mass spectrum. Since no significant signal is found, the upper limit on $\mathcal{B}\left({\Lambda}_c^{+}\to n{a}_0{(980)}^{+}\right)\times \mathcal{B}\left({a}_0{(980)}^{+}\to {\pi}^{+}\eta \right)$ is set to 8.4 × 10 − 4 at 90% confidence level. A sophisticated deep learning approach using a Transformer-based architecture is employed to distinguish signals from prevalent hadronic backgrounds, complemented by thorough validation and systematic uncertainty quantification.