First Measurement of the Absolute Branching Fraction of Λ →pμ- ν ¯ μ

Abstract

The absolute branching fraction of Λ→pμ-ν¯μ is reported for the first time based on an e+e- annihilation sample of 10×109 J/ψ events collected with the BESIII detector at s=3.097 GeV. The branching fraction is determined to be B(Λ→pμ-ν¯μ)=[1.48±0.21(stat)±0.08(syst)]×10-4, which is improved by about 30% in precision over the previous indirect measurements. Combining this result with the world average of B(Λ→pe-ν¯e), we obtain the ratio {[Γ(Λ→pμ-ν¯μ)]/[Γ(Λ→pe-ν¯e)]} to be 0.178±0.028, which agrees with the standard model prediction assuming lepton flavor universality. The asymmetry of the branching fractions of Λ→pμ-ν¯μ and Λ¯→p¯μ+νμ is also determined, and no evidence for CP violation is found.