Ultra low-mass pixel tracker for the Mu3e experiment

The Mu3e experiment is on the frontier of charged lepton flavour violating searches as well as paving the way globally in terms of cutting-edge ultra low-mass detector technologies. The search for the elusive decay of the muon to three electrons demands excellent vertexing and momentum resolution (less than 1 MeV), which is achieved using two 6 -chip inner (vertex) and two 17 or 18chip outer pixel tracking layers in the central detector region, plus two additional outer pixel layers in each of the two re-curl regions. This pixel detector exploits ultra-thin custom-designed silicon High Voltage Monolithic Active Pixel Sensors (HV-MAPS), known as MuPix. These 50 - 70 $\mu \mathrm{m}$ thick chips are mounted on low mass service flexible printed circuits (HDI), which then define the so-called “ladder” support structures of the Mu3e pixel detector. An additional $25 \mu \mathrm{m}$ thick unidirectional carbon fibre layer is added to the 18 -chip outer pixel layers. The total mass of one ladder is less than 2 grams, and the total radiation length achieved is $\mathrm{X} / \mathrm{X} 0=0.115 \%$ per layer. This talk will present an overview of the Mu3e pixel detector, in particular, preliminary results of the construction of the first ladder objects will be discussed. The Mu3e detector is expected to start data-taking mid-2025. An overview of the production procedure and status of the pixel tracker readiness will also be presented.