Search for lepton flavour violating muon decay mediated by a new light particle in the MEG experiment

Abstract

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $\hat{I}{¼}^{+}\to {\mathrm{e}}^{+}\mathrm{X},\mathrm{X}\to {\hat{I}}^3{\hat{I}}^3$. 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.