Data flow in the Mu3e data acquisition system

Abstract

The discovery of the Higgs boson by the ATLAS and Compact Muon Solenoid Experiment (CMS) experiments at CERN’s Large Hadron Collider (LHC) in 2012 marked a milestone in particle physics, completing the Standard Model of particle physics (SM) as a self-consistent theory. However, the observation of neutrino oscillations shows violations of lepton-flavour conservation, hinting at physics beyond the SM. Charged lepton-flavour violation (CLFV) decays, such as μ+ → e+e−e+, remain undetected and would be a definitive sign of new physics.

The Mu3e experiment, currently under construction at the Paul Scherrer Institute (PSI), is specifically designed to observe the decay μ+ → e+e−e+, a process exceptionally rare in the extension of the SM with neutrino oscillation, but enhanced in many theories beyond the SM. Achieving high sensitivity is crucial to detect such a rare decay. Mu3e aims for an ultimate sensitivity of one in 1016 muon decays. In its initial phase, Mu3e aims to achieve a branching ratio sensitivity of 2 x 10^−15 , analysing 1 × 10^8 muon decays per second over a year of data collection. The experiment uses highly granular detectors consisting of thin High-Voltage Monolithic Active Pixel Sensors (HV-MAPS), the MuPix chip, and scintillating timing detectors, generating approximately 100 Gbit/s of data at these particle rates.

The Mu3e data acquisition (DAQ) system, based on field programmable gate arrays (FPGAs), is a crucial component of the experiment. It employs a trigger-less readout system to deal with the randomly distributed decay particles of muons at rest. The system sorts, time-aligns, and analyses data in real time, using a filter farm of graphics processing units (GPUs) for track reconstruction.

This thesis presents the integration of subdetectors into the Mu3e DAQ system, focussing on the scintillating timing detectors, time-alignment, and data flow within the filter farm. It discusses the requirements for building the Mu3e DAQ system, including data protocols, data flow algorithms, and online data quality checks. Integration runs and testbeams at various facilities, including Deutsches Elektronen- Synchrotron (DESY), Mainz Microtron (MAMI), and PSI, have been instrumental in the refining of the system.

Moreover, this work encompasses irradiation studies of the MuPix10 chip at MAMI to understand its high-rate operation. In addition to research related to the Mu3e experiment, the development of a prototype detector for muon spin rotation (μSR) techniques using MuPix11 chips is presented. Initial tests in 2021 and a special run at PSI in 2023 demonstrated the first μSR signal detection using Si-Pixel detectors, marking the start of a novel methodology in this field.