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The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay [Formula omitted] 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 [Formula omitted] (90% CL). The combination of this result and the limit obtained by MEG gives [Formula omitted] (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.

Straw tubes with resistive cathode and cathode readout were successfully manufactured and tested. The straw tubes were manufactured by ultrasonic welding technique. The resistive cathode electrode was made of diamond like carbon (DLC). The possibility of cathode signal readout from external strip electrodes was successfully demonstrated. The event coordinate along the straw can be accurately determined by applying center of gravity method to the individual strip signals.

The Compact Linear Collider (CLIC) is an option for a future e + e - collider operating at centre-of-mass energies up to 3 TeV , providing sensitivity to a wide range of new physics phenomena and precision physics measurements at the energy frontier. This paper is the first comprehensive presentation of the Higgs physics reach of CLIC operating at three energy stages: s = 350 GeV , 1.4 and 3 TeV . The initial stage of operation allows the study of Higgs boson production in Higgsstrahlung ( e + e - → Z H ) and W W -fusion ( e + e - → H ν e ν ¯ e ), resulting in precise measurements of the production cross sections, the Higgs total decay width Γ H , and model-independent determinations of the Higgs couplings. Operation at s > 1 TeV provides high-statistics samples of Higgs bosons produced through W W -fusion, enabling tight constraints on the Higgs boson couplings. Studies of the rarer processes e + e - → t t ¯ H and e + e - → H H ν e ν ¯ e allow measurements of the top Yukawa coupling and the Higgs boson self-coupling. This paper presents detailed studies of the precision achievable with Higgs measurements at CLIC and describes the interpretation of these measurements in a global fit.

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.

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 .

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.