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 $\mathcal{B}({\Lambda }_c^{+}\to \Lambda {\pi }^{+}\eta )=(1.94\pm 0.07_{\mathrm{stat}}\pm 0.11_{\mathrm{syst}})\%$.

19 pages, 8 figures

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