The Challenge of Building Large Area, High Precision Small-Strip Thin Gap Trigger Chambers for the Upgrade of the ATLAS Experiment On behalf of the Muon System of the ATLAS Collaboration

The current innermost stations of the ATLAS muon end-cap system must be upgraded in 2019 and 2020 to retain the good precision tracking and trigger capabilities in the high background environment expected with the upcoming luminosity increase of the LHC. Large area small-strip Thin Gap Chambers up to 2 m 2 in size and totaling an active area of 1200 m 2 will be employed for fast and precise triggering. The precision reconstruction of tracks requires a spatial resolution of about 100 µm while the Level-1 trigger track segments need to be reconstructed with an angular resolution of 1 mrad. The upgraded detector will consist of eight layers each of Micromegas and small-strip Thin Gap Chamber detectors together forming the ATLAS New Small Wheels. The position of each strip must be known with an accuracy of 30 µm along the precision coordinate and 80 µm along the beam. On such large area detectors, the mechanical precision is a key point and then must be controlled and monitored all along the process of construction and integration. The precision cathode plane has strips with a 3.2 mm pitch for precision readout and the cathode plane on the other side has pads to produce a 3-out-of-4 coincidence to identify the passage of a track in an sTGC quadruplet.