A tracker for the Mu3e experiment based on high-voltage monolithic active pixel sensors

The Mu3e experiment searches for the lepton flavour violating decay μ + → e + e − e + , aiming for a branching fraction sensitivity of 10−16. This requires an excellent momentum resolution for low energy electrons, high rate capability and a large acceptance. In order to minimise multiple scattering, the amount of material has to be as small as possible. These challenges can be met with a tracker built from high-voltage monolithic active pixel sensors (HV-MAPS), which can be thinned to 50 μ m and which incorporate the complete read-out electronics on the sensor chip. To further minimise material, the sensors are supported by a mechanical structure built from 25 μ m thick Kapton foil and cooled with gaseous helium.