GEM detectors for the Upgrade of the CMS Muon Forward system

The CMS experiment is one of the two general purpose experiments at the LHC. For LHC Phase-2, the instantaneous luminosity delivered to the experiment will reach 5 $\times$ 10$^{34}$ cm$^{-2}$ s$^{-1}$, resulting in high particle fluxes that require the detectors to be upgraded. The forward regions, corresponding to the endcaps of the detectors, will receive the largest doses. In the CMS experiment, to cope with the higher event rates and larger radiation doses, triple-layer Gas Electron Multipliers (GEM) will be installed in the muon endcaps. Triple-GEM chambers will complement the existing muon system, leading to a better identification of the muon tracks and a reduction of the trigger rate due to the increase in lever arm and hence better momentum resolution. In addition, the forward coverage will be further extended. The tracker coverage will be increased up to $\lvert \eta \rvert<4$, so the muon system will also be extended from $\lvert \eta \rvert<2.4$ to the maximum possible value of $\lvert \eta \rvert<2.8$. For the inner ring of the first station of the muon endcaps, 144 GEM chambers are being built in production sites spread in 7 countries around the world. For the first time, such detectors will have large sizes of order m$^2$, thus high requirements on the uniformity across the detector are needed. Before the final installation in the CMS detector, to test their integrity, quality and performance, the GEM chambers undergo multiple quality control tests. This talk gives an introduction to GEM detectors and presents results of the performance tests.