Observation of $\Lambda_c^+ \to \Lambda a_0(980)^+$ and Evidence for $\Sigma(1380)^+$ in $\Lambda_c^+ \to \Lambda \pi^+ \eta$

Abstract

Based on $6.1~\mathrm{fb}^{-1}$ 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 $\Lambda_c^+\to\Lambda\pi^+\eta$ is performed, and branching fractions and decay asymmetry parameters of intermediate processes are determined. The process $\Lambda_c^+\to\Lambda a_0(980)^+$ is observed for the first time, and evidence for the pentaquark candidate $\Sigma(1380)^+$ decaying into $\Lambda\pi^+$ is found with statistical significance larger than $3\sigma$ with mass and width fixed to theoretical predictions. The branching fraction product $\mathcal{B}[\Lambda_{c}^{+} \to \Lambda a_0(980)^+] \; \mathcal{B}[ a_0(980)^+ \to \pi^{+}\eta]$ is determined to be $(1.05 \pm 0.16_{\mathrm{stat}} \pm 0.05_{\mathrm{syst}} \pm 0.07_{\mathrm{ext}})\%$, which is larger than theoretical calculations by 1-2 orders of magnitude. Here the third (external) systematic is from $\mathcal{B}(\Lambda_{c}^{+} \to \Lambda \pi^+ \eta)$. Finally, we precisely obtain the absolute branching fraction $\mathcal{B}(\Lambda_{c}^{+} \to \Lambda \pi^+ \eta) = (1.94 \pm 0.07_{\mathrm{stat}} \pm 0.11_{\mathrm{syst}})\%$.