Measurement of $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ and $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\pi }^0$ decays

Based on a sample of $448.1\times {10}^6\psi (3686)$ events collected with the BESIII detector, a study of $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\pi }^0$ and $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ is performed. Evidence of the isospin-violating decay $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\pi }^0$ is found for the first time with a statistical significance of $3.7\sigma$, the branching fraction $\mathcal{B}(\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\pi }^0)$ is measured to be $(1.42\pm 0.39\pm 0.59)\times {10}^{-6}$, and its corresponding upper limit is determined to be $2.47\times {10}^{-6}$ at 90% confidence level. A partial wave analysis of $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ shows that the peak around $\mathrm{\Lambda }\eta$ invariant mass threshold favors a ${\mathrm{\Lambda }}^{\ast }$ resonance with mass and width in agreement with the $\mathrm{\Lambda }(1670)$. The branching fraction of the $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ is measured to be $(2.34\pm 0.18\pm 0.52)\times {10}^{-5}$. The first uncertainties are statistical and the second are systematic.