Observation of ${\chi }_{cJ}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$

By analyzing $(448.1\pm 2.9)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector operating at the BEPCII collider, the decays of ${\chi }_{cJ}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ ($J=0$, 1 and 2) are observed for the first time with statistical significances of $13.9\sigma$, $6.7\sigma$, and $8.2\sigma$, respectively. The product branching fractions of $\psi (3686)\to \gamma {\chi }_{cJ}$ and ${\chi }_{cJ}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ are measured. Dividing by the world averages of the branching fractions of $\psi (3686)\to \gamma {\chi }_{cJ}$, the branching fractions of ${\chi }_{cJ}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ decays are determined to be $(2.31\pm 0.30\pm 0.21)\times {10}^{-4}$, $(5.86\pm 1.38\pm 0.68)\times {10}^{-5}$, and $(1.05\pm 0.21\pm 0.15)\times {10}^{-4}$ for $J=0$, 1 and 2, respectively, where the first uncertainties are statistical and the second systematic.