Pixel Sensor Evaluation and Online Event Selection for the Mu3e Experiment

Abstract

Despite having survived numerous experimental tests, the standard model of particle physics is not a complete description of nature. The Mu3e experiment tests theories beyond the standard model by searching for the lepton flavour violating decay mu->eee, aiming at a branching ratio sensitivity of 2*10^-15 in a first phase of the experiment. A high precision magnetic spectrometer combined with scintillation detectors will measure the momenta, vertices and timing of the decay products of 10^8 mu/s stopped on a target. In this work, a prototype of the high voltage monolithic active pixel sensor envisaged for the spectrometer was characterised. With an efficiency >99% and a time resolution of 14 ns, it meets the requirements imposed on the final sensor. Furthermore, an online signal selection process was developed and implemented on a graphics processing unit (GPU), keeping 98% of signal decays, while reducing the data rate of 80 Gbit/s by a factor of 140; resulting in a rate that can be stored to disk. With the computing performance achieved on the GPU, the selection process can run on the hardware planned for the experiment. Both the online selection and the silicon sensor are key aspects for the success of Mu3e.