+ ! e+
, which is predicted to occur at a sizable rate by many of new physics models
beyond the Standard Model. The upgrade expriment (MEG II) is now in preparation to
obtain one order higher branching ratio sensitivity 4 1014 by using the world's most
intense muon beam up to 108+=s at Paul Scherrer Institut.
In the MEG II experiment, Radiative Decay Counter (RDC) will be newly intro-
duced to improve the sensitivity by identifying high energy
-ray (>48 MeV) from radiative
muon decay (RMD : + ! e+e
-ray is one of the major sources of the back-
ground. According the simulation result, RDC is able to reduce background
by 54%. The improvement of the nal sensitivity is about 28 %.