A Highly Selective First-Level Muon Trigger With MDT Chamber Data for ATLAS at HL-LHC

Highly selective triggers are essential for the physics programme of the ATLAS experiment at HL-LHC where the instantaneous luminosity will be about an order of magnitude larger than the LHC design luminosity. The Level-1 muon trigger rate is dominated by low momentum muons below the nominal trigger threshold due to the limited momentum resolution of the Resistive Plate and Thin Gap trigger chambers. The resulting high trigger rates at HL-LHC can be sufficient reduced by using the data of the precision Muon Drift Tube chambers for the trigger decision. This requires the implementation of a fast MDT read-out chain and of a fast MDT track reconstruction algorithm with a latency of at most 6~$\mu$s. A hardware demonstrator of the fast read-out chain has been successfully tested at the high HL-LHC background rates at the CERN Gamma Irradiation Facility. The fast track reconstruction algorithm has been implemented on a fas trigger processor.