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The MEG II experiment, located at the Paul Scherrer Institut (PSI) in Switzerland, is the successor to the MEG experiment, which completed data taking in 2013. MEG II started fully operational data taking in 2021, with the goal of improving the sensitivity of the μ + → e + γ decay down to ∼ 6 × 10 - 14 almost an order of magnitude better than the current limit. In this paper, we describe the operation and performance of the experiment and give a new estimate of its sensitivity versus data acquisition time.

We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, μ + → e + X , X → γ γ . This search uses a dataset resulting from 7.5 × 10 14 stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013. No significant excess is found in the mass region 20–45 MeV/c 2 for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20–40 MeV/c 2 , down to O ( 10 - 11 ) at 90% confidence level.

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.

This letter reports the result of the search for the decay μ + → e + γ undertaken at the Paul Scherrer Institut in Switzerland with the MEG II experiment using the data collected in the 2021–2022 physics runs. The sensitivity of the branching ratio measurement in this search is 2.2 × 10 - 13 , a factor of 2.4 better than that of the full MEG dataset and obtained in a data taking period of about one fourth that of MEG, thanks to the superior performances of the new detector. The observed data are consistent with the expected background, yielding an upper limit on the branching ratio of B ( μ + → e + γ ) < 1.5 × 10 - 13 (90% C.L.). Additional improvements are expected with the data collected during the years 2023–2024. The data-taking will continue in the coming years.

The observation of a resonance structure in the opening angle of the electron-positron pairs in the 7 Li(p,e + e - ) 8 Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving 4 He and 12 C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the μ + → e + γ decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft–Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The γ and the e + e - pair emerging from the 8 Be ∗ transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the γ emission) of the two resonances to X17 were set, R 17.6 < 1.8 × 10 - 6 and R 18.1 < 1.2 × 10 - 5 in the mass range between 16.5 MeV / c 2 and 17.1 MeV / c 2 .

This letter reports the result of the search for the decay$${\\upmu ^+ \\rightarrow e^+ \\upgamma }$$μ + → e + γ undertaken at the Paul Scherrer Institut in Switzerland with the MEG II experiment using the data collected in the 2021–2022 physics runs. The sensitivity of the branching ratio measurement in this search is$${2.2 \\times 10^{-13}}$$2.2 × 10 - 13 , a factor of 2.4 better than that of the full MEG dataset and obtained in a data taking period of about one fourth that of MEG, thanks to the superior performances of the new detector. The observed data are consistent with the expected background, yielding an upper limit on the branching ratio of$$\\mathcal{B} ({\\upmu ^+ \\rightarrow \\textrm{e}^+ \\upgamma }) < {1.5 \\times 10^{-13}}$$B ( μ + → e + γ ) < 1.5 × 10 - 13 (90% C.L.). Additional improvements are expected with the data collected during the years 2023–2024. The data-taking will continue in the coming years.

The observation of a resonance structure in the opening angle of the electron-positron pairs in the$$^{7}$$7 Li(p,e$$^+$$+ e$$^-$$- )$$^{8}$$8 Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving$$^{4}$$4 He and$$^{12}$$12 C nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the$$\\mu ^+ \\rightarrow \\textrm{e}^+ \\gamma $$μ + → e + γ decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft–Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The$$\\gamma $$γ and the e$$^{+}$$+ e$$^{-}$$- pair emerging from the$$^8\\textrm{Be}^*$$8 Be ∗ transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the$$\\gamma $$γ emission) of the two resonances to X17 were set,$$R_{17.6} <{1.8}\\,\\times \\,10^{-6}$$R 17.6 < 1.8 × 10 - 6 and$$R_{18.1} < {1.2}\\,\\times \\,10^{-5}$$R 18.1 < 1.2 × 10 - 5 in the mass range between$${16.5}\\hbox { MeV}/\\hbox {c}^{2}$$16.5 MeV / c 2 and$${17.1}\\hbox { MeV}/\\hbox {c}^{2}$$17.1 MeV / c 2 .