Novel triple-GEM mechanical design for the CMS-ME0 detector and its preliminary performance

In the framework of the High-Luminosity Large Hadron Collider project (HL-LHC), the LHC experiments will require upgrades to their detectors to cope with the new accelerator performance. The upgrade of the CMS Muon Spectrometer foresees the installation of three new muon stations based on the Gas Electron Multiplier (GEM) technology, referred to as GE1/1, GE2/1 and ME0 detectors. While the installation and commissioning of the GE1/1 detectors is currently underway, the GE2/1 and ME0 detectors are expected to be installed between 2023 and 2027. The CMS Muon Collaboration has developed a novel construction design of large-area, trapezoidal-shaped GE1/1 triple-GEM detectors; in particular, a new self-stretching technique has been introduced to mechanically stretch the GEM-foils without using spacer grids or glue inside the gas volume in order to avoid dead regions (several percent) or possibly outgassing contaminants which could trigger premature aging processes. As it has been observed, the PCB boards, which define the gas enclosure of the detector, get deformed under the internal gas over-pressure, introducing irregularities in the planarity of the GE1/1 detector, which could potentially affect the uniformity of the detector performance. Therefore, the collaboration has established a set of tests and quality controls in order to quantify these irregularities and mitigate their impact on detector performance. Additionally, new solutions and design upgrades have been implemented to prevent such effects in future GE2/1 and ME0 upgrade projects. We will focus in particular on the novel design solutions based on the PCB distance holders (pillars), which the collaboration adopted for realization of the latter projects, and their impact on the performance of the detector, with a summary of the ongoing R&D; activities.