A search for μ+→e+γ with the first dataset of the MEG II experiment

Abstract

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay ${\mathrm{\mu }}^{+}\to {\text{e}}^{+}\mathrm{\gamma }$ from data taken in the first physics run in 2021. No excess of events over the expected background is observed, yielding an upper limit on the branching ratio of $\mathcal{B}({\mathrm{\mu }}^{+}\to {\text{e}}^{+}\mathrm{\gamma })<7.5\times {10}^{-13}$ (90% CL). The combination of this result and the limit obtained by MEG gives $\mathcal{B}({\mathrm{\mu }}^{+}\to {\text{e}}^{+}\mathrm{\gamma })<3.1\times {10}^{-13}$ (90% CL), which is the most stringent limit to date. A ten-fold larger sample of data is being collected during the years 2022–2023, and data-taking will continue in the coming years.