Measurement of Inner Bremsstrahlung in Polarized Muon Decay with MEG

A muon decay accompanied by a photon through the inner Bremmstrahlung process (mu -> e nu(nu) overbar gamma, radiative muon decay) produces a time-correlated pair of positron and photon which becomes one of the main backgrounds in the search for it mu -> e gamma decay. This channel is also an important probe of timing calibration and cross-check of whole the experiment. We identified a large sample (similar to 13000) of radiative muon decays in MEG data sample. The measured branching ratio in a region of interest in the mu -> e gamma search is consistent with the standard model prediction. It is also the first measurement of the decay from polarized muons. The precision measurement of this mode enables us to use it as one of the normalization channels of mu -> e gamma decay successfully reducing its uncertainty to less than 5%.