Speaker
Description
Mu3e is an experiment designed to search for the charged lepton flavour violating decay µ⁺ → e⁺e⁻e⁺ and it will operate in two phases. Phase I, currently under construction, will take place at the πE5 beamline at PSI, using its intense DC surface muon beam of 10⁸ µ⁺/s to reach a sensitivity of 2 × 10⁻¹⁵. Phase II will be built in the future High-Intensity Muon Beam (HIMB), to push the sensitivity to the 10⁻¹⁶ level. The nature of this decay imposes strict requirements on the detector system, especially concerning design compactness, material budget, efficiency and time resolution. This is reflected in the choice of Mupix11 as the silicon pixel sensor for the tracking system. Mupix11 is based on the HV-MAPS technology, which can deliver thin monolithic detectors while providing high performances. A first version of the Vertex detector, the innermost layers of the tracking system, has been produced and commissioned in 2025 at the PSI πE5 beamline. This campaign successfully validated several key features and proved the integration of the Vertex detector, along with other subsystems, with the high-intensity muon beamline under a 1 T magnetic field. These results represent a major milestone towards readiness for Phase I measurements. This contribution will cover the experimental design, the sensor qualification and the first results from the recent commissioning run campaign at PSI.