Observation of a near-threshold enhancement in the $\Lambda \overline{\Lambda }$ mass spectrum from ${e}^{+}{e}^{-}\to \varphi \Lambda \overline{\Lambda }$ at $\sqrt{s}$ from 3.51 to 4.60 GeV

The process ${e}^{+}{e}^{-}\to \varphi \Lambda \overline{\Lambda }$ is studied using data samples collected with the BESIII detector at the BEPCII Collider at center of mass energies $\sqrt{s}$ ranging from 3.51 GeV to 4.60 GeV. An enhancement is observed near the threshold of $\Lambda \overline{\Lambda }$. The lineshape of this enhancement is studied in different approaches, including fit with a Breit-Wigner function or a reversed exponential function. The Breit-Wigner function has a mass of $\left(2262±4±28\right)\text{}\text{}\mathrm{MeV}/{c}^{2}$ and a width of $\left(72±5±43\right)\text{}\text{}\mathrm{MeV}$, where the quoted uncertainties are statistical and systematic, respectively. The rising rate of the reversed exponential function is measured as $33±11±6\text{}\text{}\mathrm{MeV}/{c}^{2}$. For the $\Lambda \overline{\Lambda }$ system, the ${J}^{PC}$ quantum numbers of ${0}^{-+}$ and ${0}^{++}$ are rejected, while other ${J}^{PC}$ hypotheses are possible, according to the helicity-angle study. The energy-dependent cross section of the ${e}^{+}{e}^{-}\to \varphi \Lambda \overline{\Lambda }$ process is measured for the first time in this energy region, and contributions from excited $\psi $ states and vector charmoniumlike $Y$-states are investigated.