A new highly selective first level ATLAS muon trigger with MDT chamber data for HL-LHC

Felix Mueller

Max-Planck-Institut fuer Physik, Munich

Highly selective first level triggers are essential for the physics 
programme of the ATLAS experiment at the HL-LHC where the instantaneous luminosity 
will exceed the LHC's instantaneous luminosity by almost an order of magnitude. 
The ATLAS first level muon trigger rate is dominated by low momentum sub-trigger
threshold muons due to the poor momentum resolution at trigger level caused by
the moderate spatial resolution of the resistive plate and thin gap trigger chambers.
This limitation can be overcome by including the data of the precision muon drift tube
chambers in the first level trigger decision. This requires the implementation of a fast
MDT read-out chain and a fast MDT track reconstruction. A hardware demonstrator of
the fast read-out chain was successfully tested under HL-LHC operating conditions at
CERN's Gamma Irradiation Facility. It could be shown that the data provided by
the demonstrator can be processed with a fast track reconstruction algorithm on an
ARM CPU within the 6 microseconds latency of the first level ATLAS trigger anticipated
for the HL-LHC. The algorithm was also applied to ATLAS LHC run I data to prove
under realistic operating conditions that first level single muon trigger rates
as low as the high level trigger rates can be achieved with the new MDT based 
muon trigger.