We report the first measurement of the absolute branching fraction for Λc+→Λe+νe. This measurement is based on 567 pb-1 of e+e- annihilation data produced at s=4.599 GeV, which is just above the Λc+Λ¯c- threshold. The data were collected with the BESIII detector at the BEPCII storage rings. The branching fraction is determined to be B(Λc+→Λe+νe)=[3.63±0.38(stat)±0.20(syst)]%, representing a significant improvement in precision over the current indirect determination. As the branching fraction for Λc+→Λe+νe is the benchmark for those of other Λc+ semileptonic channels, our result provides a unique test of different theoretical models, which is the most stringent to date.
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© 2015 American Physical Society.