Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment
We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, μ+→e+X, X→γγ. This search uses a dataset resulting from 7.5×1014 stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009-2013. No significant excess is found in the mass region 20-45 MeV/c2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20-40 MeV/c2, down to O(10−11) at 90% confidence level.