GEM detectors characterisation for the upgrade of the CMS muon system

During its second running period (Run 2), the Large Hadron Collider (LHC) has delivered an instantaneous luminosity up to 2 × 10$^{34}$ cm $^{−2}$s$^{−1}$. In the Long Shutdown 2 (LS2) the accelerator complex will be upgraded for the next Run 3 and in preparation for the High-Luminosity LHC (HL-LHC) which will operate at instantaneous luminosities of 5 × 10$^{34}$ cm $^{−2}$s$^{−1}$ up to 7.5 × 10$^{34}$ cm $^{−2}$s $^{−1}$ starting from 2025. To handle the high expected background rates and to improve the trigger capabilities keeping the trigger rate at an acceptable level, the forward muon system of the Compact Muon Solenoid (CMS) experiment will be upgraded with Gas Electron Multiplier (GEM) and Resistive Plate Chamber (RPC) detectors. In 2019 the first endcap of triple-GEM detectors has been installed in the first station of the muon detector (GE1/1), while a the second endcap is going to be installed in 2020. A second station of triple-GEM detectors named GE2/1 will be installed in 2021–2022. We will present an overview of the CMS muon detector upgrade with triple-GEM detectors: the production and quality control of the 144 triple-GEM detectors for the first station and the final validation of the GE1/1 chambers before installation.