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

This is the authors' abstract. We don't add key points for this paper.

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