Λc → Λl+νl Form Factors and Decay Rates from Lattice QCD with Physical Quark Masses

The first lattice QCD calculation of the form factors governing Λc→Λℓ+νℓ decays is reported. The calculation was performed with two different lattice spacings and includes one ensemble with a pion mass of 139(2) MeV. The resulting predictions for the Λc→Λe+νe and Λc→Λμ+νμ decay rates divided by Vcs2 are 0.2007(71)(74) and 0.1945(69)(72) ps-1, respectively, where the two uncertainties are statistical and systematic. Taking the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vcs from a global fit and the Λc lifetime from experiments, this translates to branching fractions of B(Λc→Λe+νe)=0.0380(19)LQCD(11)τΛc and B(Λc→Λμ+νμ)=0.0369(19)LQCD(11)τΛc. These results are consistent with, and two times more precise than, the measurements performed recently by the BESIII Collaboration. Using instead the measured branching fractions together with the lattice calculation to determine the CKM matrix element gives Vcs=0.949(24)LQCD(14)τΛc(49)B.