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

We studied the radiative muon decay ^+→e^+ 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× 10^14 positive muon decays collected in the MEG experiment in the years 2009–2010 and measured the branching ratio ℬ(→e) = (6.03± 0.14(stat.)± 0.53(sys.))× 10^-8 for E_e>45 MeV and E_>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 ^+→e^+ process.