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

The current innermost stations of the ATLAS muon end-cap system must be upgraded in 2018 and 2019 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 (sTGC) 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 $\mu m$ 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 sTGC’s detectors together forming the ATLAS New Small Wheels. The position of each strip must be known with an accuracy of 40 $\mu m$ along the precision coordinate and 80 $\mu m$ 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 passage of a track in an sTGC quadruplet.