Gas electron multiplier (GEM) detectors represent a new muon system in CMS, in order to complement the existing systems in the endcaps. The forward region is the part of CMS most affected by large radiation doses and high event rates, and we foresee these parameters to be again enhanced during phase 2 of the LHC. The GEM chambers will provide additional redundancy and measurement points, allowing a better muon track identification and also wider coverage in the very forward region.

The CMS GEM detectors are made of three layers, each of which is a 50 micron thick copper-cladded polyimide foil. These chambers are filled with an Ar/CO2 gas mixture, where the primary ionisation due to incident muons will occur. The CMS GEM detectors will be the first 1-2 m^2 sized chambers of this kind, compared with previous GEM detectors. A first batch of 144 chambers will be installed during 2019-2020 in the first disk of both endcaps. Multiple quality and performance tests have already been performed, as well as first measurements with cosmic ray muons. Others chambers will follow before phase 2 of the LHC, and will be installed during 2024-2026.