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× 10¹⁴ 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 Eₑ>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.