Measurement of the Absolute Branching Fraction for $Lambda_c^+rightarrow Lambda mu^+ u_{mu}$

We report the first measurement of the absolute branching fraction for $Lambda^+_{c}rightarrow Lambda mu^+ u_{mu}$. This measurement is based on a sample of $e^+e^-$ annihilation data at a center-of-mass energy of $sqrt{s}=4.6$ GeV collected with the BESIII detector at the BEPCII storage rings. The sample corresponds to an integrated luminosity of 567 pb$^{-1}$. The branching fraction is determined to be $mathcal B({Lambda^+_crightarrow Lambda mu^+ u_{mu}})=(3.49pm0.46({rm stat})pm0.27({rm syst}))%$. In addition, we calculate the ratio $mathcal{B}(Lambda^+_crightarrow Lambda mu^+ u_{mu})/mathcal{B}(Lambda^+_crightarrow Lambda e^+ u_{e})$ to be $0.96pm0.16({rm stat})pm0.04({rm syst})$.