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, $\mu^+ \to $\mathrm{e}$^+$\mathrm{X}$, $\mathrm{X}$ \to \gamma\gamma$\mathrm{.}Thissearchusesadatasetresultingfrom$7.5\times 10^{14}$ 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.