Construction and geometrical precision assessment of the Micromegas detectors for the ATLAS New Small Wheel upgrade

The upgrade of the Large Hadron Collider (LHC) to the High Luminosity LHC (HL-LHC) is required to probe the physics beyond Standard Model. After the ongoing long shutdown (LS2), the accelerator luminosity will be increased 2 to 3 times as compared to designed luminosity value i.e. 1×〖10〗^34 〖cm〗^(-2) s^(-1). To meet the requirements of the higher rates environment of HL-LHC era, the muon system of the ATLAS detector needs to be upgraded. Therefore, the End-Cup Inner Wheel (Small Wheel) comprised of Cathode Strip Chambers (CSC) and monitored Drift Tubes (MDT) chambers will be replaced by the New Small Wheel (NSW). The NSW will be constituted by Micromegas as a primary tracking detector and small-strip Thin Gap Chambers (sTGC) for triggering. Totally 256 modules, about 3m2 each, will be installed in two wheels, 16 sectors per wheel covering a total area of ~ 1200 m2. Tracking requirement call for very high mechanical accuracy of Micromegas detector construction: typically few tens of µm over meters distance. Detector assembly methods as well as quality control tools developed are presented.