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

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay μ+ → e+ γ 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 B(μ+ → e+ γ) < 7.5×10−13 (90% CL). The combination of this result and the limit obtained by MEG gives B(μ+ → e+γ) < 3.1×10−13 (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.