GEM detectors for the Upgrade of the CMS Muon Forward system

The CMS experiment is one of the two general purpose experiments at the CERN LHC. During LHC Phase-2 starting from 2026, the instantaneous luminosity delivered to CMS will reach $5\times10^{34}$~cm$^{-2}$s$^{-1}$, resulting in high particle fluxes that require the detector to be upgraded. The forward regions, corresponding to the endcaps of the detectors, will be most affected. In the muon endcap system, triple-GEM chambers will complement the existing Cathode Strip Chambers, leading to a better identification of the muon tracks and a reduction of the trigger rate due to the suppression of fake candidates. Additionally, the forward coverage will be extended.\\ 144 GEM chambers are being built in production sites spread in 7 countries around the world for the first station of the muon endcaps. Each chamber has an area of approximately $1$ m$^2$. Thus, high requirements on the uniformity across the detector are needed and the GEM chambers undergo multiple quality control tests. In parallel with the production and testing of the chambers, 10 GEM chambers have been integrated in CMS and are currently under commissioning (slice test). Such a test is essential to prepare for the upcoming installation and integration.\\ This contribution gives an introduction to GEM chambers and presents some results of the performance tests, both for quality control and slice test.