Online Data Reduction using Track and Vertex Reconstruction on GPUs for the Mu3e Experiment

The Mu3e experiment searches for the lepton flavour violating decay μ+ → e+e−e+, aiming to achieve a sensitivity of 2 · 10−15 in its first phase and ultimately aspiring to a final sensitivity of 10−16. During the first phase of the experiment, a muon rate of ∼ 108 μ/s will be available, resulting in a data rate of ∼ 80 Gbit/s. The trigger-less readout system is based on optical links and switching FPGAs sending the complete detector data for a time slice to one node of the filter farm. A full online reconstruction is necessary to reduce the data rate to a manageable amount to be written to disk. Graphics processing units (GPUs) are used to fit tracks with a non-iterative 3D tracking algorithm for multiple scattering dominated resolution. In addition, a three track vertex selection is performed by calculating the vertex position from the intersections of the tracks. Together with kinematic cuts, this allows for a reduction of the output data rate to below 100MB/s using 12 DAQ PCs