An ultra-light helium cooled pixel detector for the Mu3e experiment

The Mu3e experiment searches for the lepton flavour violating decay μ + → e + e - e + with a target sensitivity of 1 event in 1016 decays. To achieve this goal, the experiment must minimize the material budget. The pixel detector uses High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) which are thinned down to 50 μm. Combined with gaseous helium as low density coolant, this results in only X/X 0 ≈ 0.1% per tracking layer. Both helium cooling and HV-MAPS are a novelty for particle physics experiments. Here, the work on successfully cooling a pixel tracker using gaseous helium, and performance data of the final HV-MAPS used by Mu3e, the MuPix11, is presented. The thermal studies focus on the two inner tracking layers, the Mu3e vertex detector, and the first operation of a functional thin pixel detector cooled with gaseous helium. Miniature turbo compressors are found to be sufficient to cool thin silicon pixel detectors at heat densities of up to 350 mW/cm2. The presented results demonstrate the feasibility of using HV-MAPS combined with gaseous helium as a coolant for an ultra-thin pixel detector exploring new frontiers in lepton flavor.