Observation of $\hat{I}{›}_c^{+}\to n\ddot{I}{€}^{+}\hat{I}\cdot$ and search for $\hat{I}{›}_c^{+}\to n{a}_0(980{)}^{+}$

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 $\hat{I}{›}_c^{+}\to n\ddot{I}{€}^{+}\hat{I}\cdot$ for the first time with a statistical significance of $9.5\ddot{I}ƒ$. The ratio of branching fractions $\mathcal{B}(\hat{I}{›}_c^{+}\to n\ddot{I}{€}^{+}\hat{I}\cdot )/\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›\ddot{I}{€}^{+}\hat{I}\cdot )$ is measured to be $0.155\pm 0.031_{stat.}\pm 0.012_{syst.}$ Taking the world average of $\mathcal{B}(\hat{I}{›}_c^{+}\to \hat{I}›\ddot{I}{€}^{+}\hat{I}\cdot )$ as reference, the absolute branching fraction is calculated to be $\mathcal{B}(\hat{I}{›}_c^{+}\to n\ddot{I}{€}^{+}\hat{I}\cdot )=(2.94\pm 0.59_{stat.}\pm 0.23_{syst.}\pm 0.13_{ref.})\times 10^{-3}$. The intermediate process $\hat{I}{›}_c^{+}\to n{a}_0(980{)}^{+}$ is also searched for in the $\ddot{I}{€}^{+}\hat{I}\cdot$ invariant mass spectrum. Since no significant signal is found, the upper limit on $\mathcal{B}(\hat{I}{›}_c^{+}\to n{a}_0(980{)}^{+})\times \mathcal{B}(a_0(980{)}^{+}\to \ddot{I}{€}^{+}\hat{I}\cdot )$ 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.

25 pages, 6 figures