A First-Level Muon Trigger Based on the ATLAS Muon Drift Tube Chambers With High Momentum Resolution for LHC Phase II

The Level-1 (L1) trigger for muons with high transverse momentum (pT) in ATLAS is based on chambers with excellent time resolution, able to identify muons coming from a particular beam crossing. These trigger chambers also provide a fast pT-measurement of the muons, the accuracy of the measurement being limited by the moderate spatial resolution of the chambers along the deflecting direction of the magnetic field (eta-coordinate). The higher luminosity foreseen for Phase-II puts stringent limits on the L1 trigger rates, and a way to control these rates would be to improve the spatial resolution of the triggering system, drastically sharpening the turn-on curve of the L1 trigger. To do this, the precision tracking chambers (MDT) can be used in the L1 trigger, provided the corresponding trigger latency is increased as foreseen. 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 w.r.t. the relevant beam crossing and in a sufficiently fast way to fit into the available L1 latency. The adaption to chamber geometries in barrel and end-cap will also be discussed. 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, simulation results are shown which demonstrate the rejection efficiency for muons below a given pT-threshold, taking into account deteriorating effects like delta-rays, conversion background and tube inefficiencies.