Study of ${\Lambda }_{c}^{+}\to \Lambda {\mu }^{+}{\nu }_{\mu }$ and test of lepton flavor universality with ${\Lambda }_{c}^{+}\to \Lambda {\ell }^{+}{\nu }_{\ell }$ decays

The study of the Cabibbo-favored semileptonic decay ${\Lambda }_{c}^{+}\to \Lambda {\mu }^{+}{\nu }_{\mu }$ is reported using $4.5\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ annihilation data collected at center-of-mass energies ranging from 4.600 to 4.699 GeV. The branching fraction of the decay is measured to be $B\left({\Lambda }_{c}^{+}\to \Lambda {\mu }^{+}{\nu }_{\mu }\right)=\left(3.48±0.1{4}_{\mathrm{stat}}±0.1{0}_{\mathrm{syst}}\right)%$, 3 times more precise than the prior world average result. Tests of lepton flavor universality using ${\Lambda }_{c}^{+}\to \Lambda {\ell }^{+}{\nu }_{\ell }$ ($\ell =e$, $\mu $) decays are reported for the first time, based on measurements of the differential decay rates and the forward-backward asymmetries in separate four-momentum transfer regions. The results are compatible with Standard Model predictions. Furthermore, we improve the determination of the form-factor parameters in ${\Lambda }_{c}^{+}\to \Lambda {\ell }^{+}{\nu }_{\ell }$ decays, which provide stringent tests and calibration for lattice quantum chromodynamics calculations.