Measurement of the Radiative Decay Branching Fraction in the MEG Experiment

Book excerpt: A measurement of the radiative muon decay [mu] --> e v e̳ v [m̳u̳] [gamma] branching fraction was done on the basis of the radiative decay data of MEG experiment. The data were taken periodically one day per week with the beam intensity of 1.2x106 [mu] + / s from September to December, 2008. The positron was measured by a spectrometer with gradient magnetic field. The photon was detected by an innovative ̃900 liter liquid xenon scintillation detector. The measurement was carried out with a cut-and-count approach. We report the radiative muon decay branching fraction to be B([mu] --> e vv̄ [gamma]) = (2.84 ± 0.20(stat) ± 0.05(syst))x10−7, with Ee̳> 46 MeV, E [gamma]> 30 MeV, and the opening angle between the two particles [theta] e̳ [gamma] in its full kinematic range. Distributions of three observables, E e̳, E [gamma], and [theta] e̳ [gamma], as well as their kinematic correlations were verified to be consistent with the radiative decays. The measurement is in excellent agreement with the prediction of the Standard Model.