Development of a Concept for the Muon Trigger of the ATLAS Detector at the HL-LHC

Highly selective first level triggers are essential to exploit the full physics potential of the ATLAS experiment at the High Luminosity-Large Hadron Collider, where the instantaneous luminosity will exceed the LHC Run 1 instantaneous luminosity by almost an order of magnitude. The ATLAS experiment plans to increase the rate of the first trigger level to 1 MHz at 6 µs latency. The momentum resolution of the existing first level muon trigger is limited by the moderate position resolution of the trigger chambers. Including the data of the precision Monitored Drift Tube (MDT) chambers in the first level muon trigger decision will increase the selectivity of the first level muon trigger substantially. Run 1 LHC data with a centre-of-mass energy of $\sqrt{s} = 8\, \textrm{TeV}$ and a bunch spacing of 25 ns was used to study the achievable selectivity of a muon trigger making use of the MDT data. It could be shown that it is not necessary to fully reconstruct the muon trajectory. The position and direction information of the straight track segments reconstructed in the MDT chambers is sufficient to measure the momentum with a precision that allows for a rate reduction compared to the expected Phase-I trigger rate of over 70 % for the whole ATLAS muon spectrometer. Fast algorithms employed in the trigger electronics are required for the reconstruction of the track segments within the trigger latency. For the end-cap ($1.05 < |\eta| < 2.4$) the ATLAS collaboration considered a 1-dimensional Hough transform algorithm, which is seeded by the trigger chamber data. The algorithm is not applicable in the barrel region ($0 < |\eta| < 1.05$) because of the lower spatial resolution of the trigger chambers in the barrel region than in the end-cap region. Extending the algorithm to a Binned 2D-Hough Transform, which improves the track segment reconstruction quality sufficiently for all regions apart from the outer barrel MDT chambers. In this thesis, a new track segment finding algorithm, that makes use of tangents to drift radii, was developed and shown to be applicable to the entire muon spectrometer ($ |\eta| < 2.4$).