Abstract
We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, upmu ^+ rightarrow mathrm {e}^+mathrm {X}, mathrm {X} rightarrow upgamma upgamma . This search uses a dataset resulting from 7.5times 10^{14} 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/c^2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c^2, down to {mathcal {O}}(10^{-11}) at 90% confidence level.
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