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

Using data samples with a total integrated luminosity of approximately 18 fb${}^{-1}$ collected by the BESIII detector operating at the BEPCII, the process $e^{+}{e}^{-}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ is studied at center-of-mass energies between 3.5106 and 4.6988 GeV. The Born cross section for the process $e^{+}{e}^{-}\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}\eta$ is measured. No significant structure is observed in the Born cross section line shape. An enhancement near the $\mathrm{\Lambda }\bar{\mathrm{\Lambda }}$ 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 $\mathrm{\Lambda }$ states and potential interferences, the mass and width are determined to be ($2356\pm 7\pm 17$) MeV/$c^2$ and ($304\pm 28\pm 54$) MeV, respectively, where the first uncertainties are statistical and the second are systematic.