ATLAS L1 Muon Trigger Upgrade with sTGC: Design and Performance

We describe the upgrade of the ATLAS forward Level 1 (L1) muon trigger planned for the LHC run with luminosity above 2 10 34 cm. This upgrade, which aims at suppressing the fake muon triggers from non-pointing tracks, foresees the installation of a New Small Wheel (NSW) detector in the endcap region. This region of the detector will be instrumented with small-strip Thin Gap Chambers (sTGC) that will allow to keep the L1 muon trigger rate below 25 kHz. This rate suppression is realized with a two-step trigger system: first, an ultra-fast pad trigger defines the regions of interest containing potential high- p T muon candidates; second, an accurate track measurement is performed with precision readouts from the sTGC strips, providing the required 1 mrad angular resolution. The new, sTGC-based, L1 muon trigger is reviewed. A description of the sTGC detector as well as of its readout system is given. The first results from the simulation of this new trigger system are presented. These studies show that the pad-trigger system is almost 100% efficient, and that the angular resolution achieved with the sTGC measurements strips is better than 1 mrad, giving an overall sTGC trigger efficiency close to 93%. The system performance with the pileup and background rates expected for the high-luminosity LHC are presented as well, including an estimate of the fake muon track rate