A search for $\mu^+\to e^+\gamma$ with the first dataset of the MEG II experiment

TL;DR

The MEG II experiment conducted its first search for the rare decay $^+ o e^+\u03b3$ and set the most stringent upper limit to date, with ongoing data collection expected to improve sensitivity.

Contribution

First results from the MEG II experiment's search for $^+ o e^+\u03b3$, establishing the most stringent upper limit on its branching ratio to date.

Findings

No excess events observed over background.

Upper limit on branching ratio: < $7.5 imes 10^{-13}$.

Combined limit with previous MEG data: < $3.1 imes 10^{-13}$.

Abstract

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay $\mu^+\to e^+\gamma$ 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 B($\mu^+\to e^+\gamma$) < $7.5 \times 10^{-13}$ (90% C.L.). The combination of this result and the limit obtained by MEG gives B($\mu^+\to e^+\gamma$) < $3.1 \times 10^{-13}$ (90% C.L.), 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.