Search for the lepton flavour violating decay $$\mu ^+ \rightarrow \mathrm {e}^+ \gamma $$ µ + ? e + ? with the full dataset of the MEG experimentMEG Collaboration

The final results of the search for the lepton flavour violating decay $$\mathrm {\mu }^+ \rightarrow \mathrm {e}^+ \mathrm {\gamma }$$ based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling $$7.5\times 10^{14}$$ stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of $$ \mathcal{B} (\mu ^+ \rightarrow \mathrm{e}^+ \gamma ) < 4.2 \times 10^{-13}$$ (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.