Gas electron multipliers (GEMs) are a new addition to the CMS muon system. They complement existing detectors in the forward regions close to the beampipe, where large radiation doses and high event rates will increase during Phase-2 of the LHC. The harsh environment imposes strict requirements on detector characteristics, requiring technologies with high rate capabilities–hence the choice of GEMs. In the endcaps, the GEM system will improve the measurement of the polar muon bending angle, allowing the trigger of the muon system to cope with the high rates. Additionally, GEMs will further extend the muon system coverage in the very forward regions.

Like the other CMS muon subsystems, GEMs are gaseous detectors. Their key feature is the GEM foil, which consists of a 50 micrometer-thick insulating polymer (polyimide) surrounded on the top and bottom with copper conductors. Throughout the foil, microscopic holes are etched in a regular hexagonal pattern. The CMS GEM chambers consist of two PCBs, containing the gas volume, and a stack of three GEM foils in between. The chambers are filled with an Ar/CO2 gas mixture, which is ionized by incident ionizing particles. A potential difference applied across the foils generates sharp electric fields in the holes. The electrons created during the ionisation process drift towards the foils and are multiplied in the holes. The resulting electron avalanche induces a readout signal on the finely spaced strips.

The CMS GEMs are the first GEM chambers with such a large size (an area of about 0.5 m^2) and the largest GEM system ever installed. A first batch of 144 chambers was installed during Long Shutdown 2 on the first disk of the two endcaps. These chambers will contribute to data-taking from Run-3 of the LHC. Additionally, two more disks of GEM chambers will be installed in each endcap during 2024-2026, before Phase-2 of the LHC.…

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