A thin silicon pixel tracker for the Mu3e experiment

The Mu3e experiment will search for the charged lepton flavor violating decay μ → eee, which is suppressed to unobservable levels in the Standard Model. The aim is to either discover it, which would be a clear sign of new pyhsics, or to exclude it above a branching ratio of 10^-16 at 90% CL. To achieve that, the world's highest intensity muon beam is required, which will be available at the Paul Scherrer Institute (PSI) in Switzerland. The experiment is based on low energy muons being stopped on a target and decaying at rest, which results in decay electrons with energies ≤ 53 MeV. Backgrounds can be suppressed by very precise momentum, vertex and timing measurements. Due to the electrons' low energies, the vertex and momentum resolution are dominated by multiple scattering, which makes a low material budget in the active detector region crucial for the experiment. The other basic requirement is the capability of dealing with muon decay rates of about 10⁹ (1)/(s). A barrel shaped detector system has been designed which consists of four layers of high-voltage monolithic active pixel sensors (HV-MAPS) in a 1T solenoid magnet for tracking and a combination of scintillating fibers and tiles for additional timing measurements. After a general overview of the Mu3e experiment, this talk will focus on the pixel tracker including the sensors, readout scheme, mechanics and cooling. Performance results of the last sensor prototype, the MuPix7, are presented.