The first level muon trigger of the ATLAS experiment will be upgraded to operate at the High-Luminosity LHC. The selectivity of the current system is limited by the moderate spatial resolution of RPC and TGC. The MDT chambers currently used for precision tracking will be therefore included to improve the momentum resolution and the redundancy.
In the upgraded muon trigger system, the MDT trigger processors will receive MDT hits from the detectors and match them to the trigger candidates (seeds) from the RPC and TGC trigger systems. These seeds provide a Region-of-Interest (RoI) and the bunch-crossing timing which is used for calculating the MDT drift time. MDT hits matched to the RoI are then used by the MDT trigger algorithm to improve the momentum resolution, by forming track segments and joining them together for momentum determination.
A hardware demonstrator of the MDT trigger processor, based on a common ATCA platform known as "APOLLO", is currently under production. It consists of two separate modules called the "Command Module" and "Service Module", and it is based on FPGA technology.
A description of the algorithms for the MDT track reconstruction is presented. The achieved trigger performance allows to reconstruct muon tracks with a momentum resolution of about 6% and a trigger efficiency above 95% for muons with a transverse momentum of 20 GeV.