Measurement of the radiative decay of polarized muons in the MEG experiment

We studied the radiative muon decay μ+→e+νν¯γ by using for the first time an almost fully polarized muon source. We identified a large sample (∼13,000) of these decays in a total sample of 1.8×1014 positive muon decays collected in the MEG experiment in the years 2009–2010 and measured the branching ratio B(μ→eνν¯γ)=(6.03±0.14(stat.)±0.53(sys.))×10−8 for Ee>45 MeV and Eγ>40 MeV, consistent with the Standard Model prediction. The precise measurement of this decay mode provides a basic tool for the timing calibration, a normalization channel, and a strong quality check of the complete MEG experiment in the search for μ+→e+γ process.