Status of the MEG II experiment

Abstract

The MEG II experiment, which focuses on investigating Charged Lepton Flavour Violation in muon decays, completed the commissioning of all subdetectors in time for the 2021 run and it is currently collecting its third year of beam time at the Paul Scherrer Institut (CH). The experimental apparatus has been specifically designed to search for 𝜇 + → 𝑒 + 𝛾 decay, aiming at improving the limit on the branching ratio from the current 4.2 × 10 −13 at 90% confidence level set by the former MEG experiment. This requires high-performance and lightweight detectors capable of handling the pileup effect from world most intense continuous 𝜇 + beam. This contribution provides a brief overview of the experimental techniques employed and then focuses on the description of the collected datasets and analysis strategy. Although we accumulated only a few weeks of data in 2021, the experiment already has a sensitivity of 8.8 × 10 −13 to the 𝜇 + → 𝑒 + 𝛾 branching ratio, with analysis strongly limited by available statistics. In the months following the conference, that dataset was unblinded and the limit was set at 7.5 × 10 −13 . The combination of this with the existing limit yields the current best exclusion of 𝜇 + → 𝑒 + 𝛾 process with a limit in branching ratio of 3.1 × 10 −13 at 90% confidence level. Finally, MEG II data-taking is expected to continue until the end of 2026, to achieve its final sensitivity of 6 × 10 −14 .