Cosmic ray validation of electrode positions in small-strip thin gap chambers for the upgrade of the ATLAS detector

The Large Hadron Collider (LHC) is used to generate subatomic physics processes at the energy frontier to challenge our understanding of the Standard Model of particle physics. The particle collision rate at the LHC will be increased up to seven times its design value in 2025-2027 by an extensive upgrade program. The innermost endcaps of the ATLAS muon spectrometer consist of two wheels of muon detectors that must be replaced to maintain the muon momentum resolution in the high-rate environment. The so-called New Small Wheels (NSWs) are made of two detector technologies: micromegas and small-strip thin gap chambers (sTGCs). The sTGCs are gas ionization chambers that hold a thin volume of gas between two cathode boards. One board is segmented into copper readout strips of 3.2 mm pitch that are used to precisely reconstruct the coordinate of a passing muon. Modules of four sTGCs glued together into quadruplets cover the NSWs. Quadruplets were designed to achieve a 1 mrad angular resolution to fulfill the spectrometer's triggering and precision tracking requirements. To achieve the required angular resolution the absolute position of the readout strips must be known in the ATLAS coordinate system to within 100 μm. At McGill University, the performance of sTGC quadruplets was characterized using cosmic ray data before being sent to CERN, where the charge profile left by x-rays is used to measure the offset of the strip patterns with respect to nominal at a limited number of points on the surface of each quadruplet. The x-ray strip position measurements have acceptable but limited precision and do not span the whole area of the strip layers. Given the importance of knowing the absolute position of each readout strip to achieve the performance requirements of the NSWs, the x-ray method must be validated by an independent method. Cosmic ray data is used to characterize the relative alignment between layers and validate the x-ray method.