The Level-1 (L1) trigger for muons with high transverse momentum (pT) in ATLAS is based on chambers with excellent time resolution (better than 20 ns), able to identify muons coming from a particular beam crossing. About 600 of these trigger chambers are located in the central region (eta<1) of ATLAS, while about 3600 are covering the forward region (1<eta<2.4). In the central and forward region the chamber technology is of the RPC and TGC type, respectively. A detailed description of the present ATLAS trigger scheme and chamber technologies is given in reference .
The 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 limited momentum resolution of the trigger chambers weakens the selectivity of the L1 trigger for high-pT muons above a predefined threshold, like 20 GeV, allowing muons below threshold to cause “fake” triggers, mostly corresponding to event signatures without physics interest.
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 with respect to pT. This is possible due to the close matching between trigger and MDT precision chambers in the Muon spectrometer. The selectivity for high-pT tracks can thus be improved by combining the excellent spatial resolution of the MDT with the time resolution of the trigger chambers.
In this concept, the trigger chambers will be used to define regions of Interest (RoI) inside which high-pT muon candidates have been identified. MDT hits in the RoI(s) are passed to the trigger logic, where they are used for an accurate estimate of the track momentum, leading to an efficient suppression of sub-threshold muon triggers. In order to collect the MDT hit coordinates early enough for use in the L1 trigger logic, the relevant hits are read out through a priority readout chain, independent of the standard, asynchronous readout. Considering only MDT hits inside the RoI(s) strongly reduces bandwidth requirements and latencies, in such a way that transfer and processing of the MDT for the L1 trigger decision can be accomplished within the 20 mysec L1 latency, available in Phase-II.
We present the architecture of the MDT trigger system together with estimates of latency and spatial resolution as well as test results from a prototype, performed at the CERN Gamma Irradiation Facility (GIF) early this year. We also present results from a demonstrator module, containing all essential components of the readout system to be used in Phase-II.
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.
 ATLAS collaboration, The ATLAS experiment at the CERN LHC, 2008 JINST 3 S08003