Concept of a Stand-Alone Muon Trigger with High Transverse Momentum Resolution for the ATLAS Detector at the High-Luminosity LHC

The ATLAS trigger uses a three-level trigger system. The level-1 (L1) trigger for muons with high transverse momentum pT in ATLAS is based on fast chambers with excellent time resolution which are able to identify muons coming from a particular beam crossing. These trigger chambers also provide a fast measurement of the muon transverse momenta, however with limited accuracy caused by the moderate spatial resolution along the deflecting direction of the magnetic field. The higher luminosity foreseen for Phase-II puts stringent limits on the L1 trigger rates. A way to control these rates is the improvement of the spatial resolution of the triggering device which drastically sharpens the turn-on curve of the L1 trigger. To do this, the precision tracking chambers (MDT) can be used in the L1 trigger, if the corresponding trigger latency is increased as planned. The trigger rate reduction is accomplished by strongly decreasing the rate of triggers from muons with pT lower than a predefined threshold (typically 20 GeV), which would otherwise trigger the DAQ. We describe the architecture for reading out the MDT synchronously with respect to the relevant beam crossing and in a sufficiently fast way to fit into the available L1 latency. We present results of a prototype test at the Gamma Irradiation Facility (GIF) at CERN as well as the performance of a demonstrator module, containing most of the required functionality. In addition, results obtained with experimental and simulated data are shown which demonstrate the rejection efficiency for muons below a given pT threshold, taking into account degrading effects like delta rays, conversion background and tube inefficiencies.