  The Mu3e experiment searches for the lepton flavour violating decay mu+ ->
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 minimize 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 um 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 um thick Kapton foil and
cooled with gaseous helium.
