FOR 5199: Searching for charged lepton flavour violation with the Mu3e experiment

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The project aims to set up a DFG-Research Unit for all German groups participating in the Mu3e experiment (Heidelberg, Karlsruhe and Mainz).With the Mu3e experiment we aim to search for the rare decay μ → eee with an ultimate sensitivity of less than B(μ → eee) <1E-16. This decay is allowed in the extended Standard Model (SM) of particle physics (with massive neutrinos), albeitan extremely small decay rate of << 1E-56 is predicted, which is why this decay is within the SM virtually unobservable . The discovery of this lepton flavor violating (LFV) decay would therefore manifest the existence of new physics beyond the SM.LFV processes are predicted by a variety of models beyond the SM. These include supersymmetry, Little Higgs models, models with extra (compactified) space dimensions, or grand unification theories. It is therefore not surprising that several groups around the world are preparing different experiments to search for LFV processes.The current experimental exclusion limit for the decay μ → eee comes from the SINDRUM experiment, which was performed more than 30 years ago, and is B(μ → eee) <1E-12 (90% confidence level). The Mu3e experiment thus ultimately seeks to improve the sensitivity by four orders of magnitude.In a first phase (Phase I), the Mu3e experiment will be performed at the PiE5 beamline at the Paul-Scherrer Institute (PSI, Switzerland), which provides a pure high-intensity muon beam. For phase I, a sensitivity of B(μ → eee)<5E-15 is expected after about 3 years of running time. Phase II will require a new, even more intense beamline, which is currently in the planning stage.Within the proposed Research Unit the German Mu3e groups plan to setup, commission and operate the Mu3e Phase I experiment. The German groups have been instrumental in the preparation of the experiment and plan to take responsibility for major components of the detector. These include the tracking detector build from modern monolithic silicon pixel sensors, the scintillation counters for precise timing, and the data acquisition system including a GPU-based processor farm.