First Measurement of the Absolute Branching Fraction of Λ→pμ^{-}ν[over ¯]_{μ}

Area of Science:

• Particle Physics
• Hadron Spectroscopy
• Electroweak Interactions

Background:

• The study of hyperon decays provides crucial tests of the Standard Model.
• Previous measurements of the Lambda to proton, muon, and neutrino decay branching fraction relied on indirect methods.
• Understanding lepton flavor universality is a key aspect of fundamental physics.

Purpose of the Study:

• To report the first direct measurement of the absolute branching fraction of the decay Lambda -> p mu- nu_bar.
• To test lepton flavor universality by comparing the muon and electron decay modes of the Lambda baryon.
• To search for CP violation in Lambda decays.

Main Methods:

• Analysis of 10x10^9 J/psi events collected by the BESIII detector.
• Utilizing e+e- annihilation data at sqrt(s) = 3.097 GeV.
• Precise determination of branching fractions and asymmetry parameters.

Main Results:

• The absolute branching fraction B(Lambda -> p mu- nu_bar) is measured to be (1.48 +/- 0.21(stat) +/- 0.08(syst)) x 10^-4.
• The ratio of branching fractions [Gamma(Lambda -> p mu- nu_bar)] / [Gamma(Lambda -> p e- nu_bar)] is found to be 0.178 +/- 0.028.
• No evidence for CP violation was observed in the measured decay channels.

Conclusions:

• The measured branching fraction offers a 30% improvement in precision over previous indirect measurements.
• The results are consistent with the Standard Model prediction for lepton flavor universality.
• The study sets stringent limits on CP violation in Lambda decays.