Measurement of ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }\eta $ from 3.5106 to 4.6988 GeV and study of $\Lambda \overline{\Lambda }$ mass threshold enhancement

Using data samples with a total integrated luminosity of approximately $18\text{}\text{}{\mathrm{fb}}^{-1}$ collected by the BESIII detector operating at the Beijing Electron-Positron Collider II, the process ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }\eta $ is studied at center-of-mass energies between 3.5106 and 4.6988 GeV. The Born cross section for the process ${e}^{+}{e}^{-}\to \Lambda \overline{\Lambda }\eta $ is measured. No significant structure is observed in the Born cross section line shape. An enhancement near the $\Lambda \overline{\Lambda }$ mass threshold is observed for the first time in the process. The structure can be described by an $S$-wave Breit-Wigner function. Neglecting contribution of excited $\Lambda $ states and potential interferences, the mass and width are determined to be $\left(2356±7±15\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$ and $\left(304±28±54\right)\text{}\text{}\mathrm{MeV}$, respectively, where the first uncertainties are statistical and the second are systematic.