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 X 10^34 cm-2s-1. To meet the requirements of higher rates environment of HL-LHC era, the muon system of ATLAS detector needs to be upgraded. Therefore, the 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 Micromesh Gaseous Structure (Micromegas) detectors and small-strip Thin Gap Chambers (sTGC). Micromegas detectors will be used for tracking as well as triggering purpose. In each of the NSW (A&C), 16 modules will be installed in 16 sectors i.e. 8 large sectors and 8 small sectors; covering total area of ~ 1200 m2. Micromegas are ionizationbased gaseous detectors made up of parallel plates, having a thin amplification region separated from the conversion region via thin metallic mesh. The production of Micromegas detectors as well as methods adopted to achieve required geometrical precision are presented. Specific measurement devices have been developed in the last few years to determine the quality of Micromegas chambers required for NSW. Planarity measurements of drift, readout panels as well as modules after assembly are done with a specific CMM (co-ordinate measuring machine), results are shown for drift, readout panels as well as modules. Results of in-plane measurements (XY co-ordinate), performed using Rasnik masks etched on the PCBs are also reported. Modules constructed using the dedicated tools and methods are validated using cosmic muons, results of one such module are shown.