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

Abstract

We studied the radiative muon decay ${\mu }^{+}\to e^{+}\nu \bar{\nu }\gamma$ by using for the first time an almost fully polarized muon source. We identified a large sample (~13000) of these decays in a total sample of 1.8x10^14 positive muon decays collected in the MEG experiment in the years 2009--2010 and measured the branching ratio B(${\mu }^{+}\to e^{+}\nu \bar{\nu }\gamma$) = (6.03+-0.14(stat.)+-0.53(sys.))x10^-8 for E_e > 45 MeV and E_{\gamma} > 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 ${\mu }^{+}\to e^{+}\gamma$ process.

Comments: 8 pages, 7 figures. Added an introduction to NLO calculation which was recently calculated. Published version