Measurement of the absolute branching fraction for Λc+→Λμ+νμ

We report the first measurement of the absolute branching fraction for Λc+→Λμ+νμ . This measurement is based on a sample of e+e− annihilation data produced at a center-of-mass energy 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 B(Λc+→Λμ+νμ)=(3.49±0.46(stat)±0.27(syst))% . In addition, we calculate the ratio B(Λc+→Λμ+νμ)/B(Λc+→Λe+νe) to be 0.96±0.16(stat)±0.04(syst) .