Observation of ${\Lambda }_c^{+}\to n{\pi }^{+}\eta$ and search for ${\Lambda }_c^{+}\to n{a}_0(980{)}^{+}$

Abstract

By analysing 6.1 ${\mathrm{fb}}^{-1}$ of data collected at center-of-mass energies between $\sqrt{s}=4.600$ and 4.843 $\mathrm{GeV}$ 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 $9.5\sigma$. The ratio of branching fractions $\mathcal{B}({\Lambda }_c^{+}\to n{\pi }^{+}\eta )/\mathcal{B}({\Lambda }_c^{+}\to \Lambda {\pi }^{+}\eta )$ is measured to be $0.155\pm 0.031_{stat.}\pm 0.012_{syst.}$ Taking the world average of $\mathcal{B}({\Lambda }_c^{+}\to \Lambda {\pi }^{+}\eta )$ as reference, the absolute branching fraction is calculated to be $\mathcal{B}({\Lambda }_c^{+}\to n{\pi }^{+}\eta )=(2.94\pm 0.59_{stat.}\pm 0.23_{syst.}\pm 0.13_{ref.})\times 10^{-3}$. The intermediate process ${\Lambda }_c^{+}\to n{a}_0(980{)}^{+}$ is also searched for in the ${\pi }^{+}\eta$ invariant mass spectrum. Since no significant signal is found, the upper limit on $\mathcal{B}({\Lambda }_c^{+}\to n{a}_0(980{)}^{+})\times \mathcal{B}(a_0(980{)}^{+}\to {\pi }^{+}\eta )$ is set to $8.4\times 10^{-4}$ 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.

26 pages, 6 figures

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