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, $$\upmu ^+ \rightarrow \mathrm {e}^+\mathrm {X}, \mathrm {X} \rightarrow \upgamma \upgamma \ $$ . This search uses a dataset resulting from $$7.5\times 10^{14}$$ 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.