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Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay µ + → e + e − e + . Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on 50 µm thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1 T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25 Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is ≥ 99% in a high rate test beam.

The MuPix7 chip is a monolithic HV-CMOS pixel chip, thinned down to 50 µm. It provides continuous self-triggered, non-shuttered readout at rates up to 30 Mhits/chip of 3 × 3 mm 2 active area and a pixel size of 103 × 80 µm 2 . The hit efficiency depends on the chosen working point. Settings with a power consumption of 300 mW/ cm 2 allow for a hit efficiency > 99.5%. A time resolution of 14.2 ns (Gaussian sigma) is achieved. Latest results from 2016 test beam campaigns are shown.

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Mu3e is an upcoming experiment at Paul Scherrer Institut in the search for the strongly suppressed decay of µ → eee. It will use an ultra-lightweight silicon pixel detector using thinned HV-CMOS MAPS chips. Multiple Coulomb scattering is further kept under control with using high density interconnects made of aluminium and operating the detector in a helium atmosphere. More than 1 m 2 of instrumented surface will produce about 3.3 kW of heat (≤ 250 mW/cm 2). Traditional cooling approaches are in conflict with the low-mass requirements, hence a gaseous helium flow cooling system will be implemented. This talk will give a report on the successful data transmission tests with the aluminium interconnects at target speeds of 1.25 Gbit/s under realistic condition. The final proof-of-concept of the helium cooling has been achieved with comprehensive cooling simulations and successfully confirmed with laboratory measurements using a full-scale mock-up of the vertex pixel detector.