Observation of $Λ^+_c\to nπ^+η$ and search for $Λ^+_c\to na_0(980)^+$

Abstract

By analysing 6.1 ${\rm fb}^{-1}$ of data collected at center-of-mass energies between $\sqrt{s}=4.600$ and 4.843 $\rm GeV$ with the BESIII detector at the BEPCII collider, we observe the decay $Λ_c^+\to nπ^+η$ for the first time with a statistical significance of $9.5σ$. The ratio of branching fractions $\mathcal{B}(Λ_c^+\to nπ^+η)/\mathcal{B}(Λ_c^+\to Λπ^+η)$ is measured to be $0.155\pm0.031_{\rm stat.}\pm0.012_{\rm syst.}$ Taking the world average of $\mathcal{B}(Λ_c^+\to Λπ^+η)$ as reference, the absolute branching fraction is calculated to be $\mathcal{B}(Λ_c^+\to nπ^+η)=(2.94\pm0.59_{\rm stat.}\pm0.23_{\rm syst.}\pm0.13_{\rm ref.})\times10^{-3}$. The intermediate process $Λ_c^+\to na_0(980)^+$ is also searched for in the $π^+η$ invariant mass spectrum. Since no significant signal is found, the upper limit on $\mathcal{B}(Λ_c^+\to na_0(980)^+)\times\mathcal{B}(a_0(980)^+\toπ^+η)$ is set to $8.4\times10^{-4}$ 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.

Figures (6)

• Figure 1: Topological diagrams for $\Lambda_c^+\to n a_0(980)^+$ assuming $a_0(980)^+$ as a tetraquark state.
• Figure 2: Topological diagrams for the $\Lambda_c^+\to n\pi^+\eta$ process, where (a--c) receive contributions from $H(\bar{\textbf{6}})$, while (d) and (e) receive contributions from $H(\textbf{15})$.
• Figure 3: The $M_{\rm{miss}}$ distributions for the $\eta\to\gamma\gamma$ and $\eta\to\pi^+\pi^-\pi^0$ channels after veto cuts. The points with error bars represent data. The histograms with filled colors represent different background components in the inclusive MC sample. The histograms with blank fillings indicate arbitrarily scaled signal shapes.
• Figure 4: Post-fit $M_{\rm{miss}}$ spectra of data samples. The points with error bars represent data. The red solid lines represent the total fit results. The dashed areas represent signal components and the dashed lines represent background components.
• Figure 5: Fit to the $M_{\pi^+\eta}$ distribution. The black points with error bars represent data. The red line represents the fit result. The green line represents the total $n\pi^+\eta$ signal. The blue line represents the non-resonant $\pi^+\eta$ signal. The red dashed area represents the $na_0(980)^+$ signal, and the violet line is the total background including $\Lambda\pi^+\eta$, $\Sigma^0\pi^+\eta$, other $\Lambda_c^+$ background, and non-$\Lambda_c^+$ background.