GEM detectors for the CMS Endcap Muon System: status of three detector stations

The High-Luminosity Large Hadron Collider (HL-LHC), due to start operations in 2029, aims to achieve instantaneous luminosities 5 to 7.5 times higher than the LHC nominal value. To effectively address the muon triggering and reconstruction performance under the significantly increased background levels, the CMS collaboration is conducting several upgrades of the present systems. Among these, the Gas Electron Multiplier (GEM) detector project aims to instrument the very forward region of the CMS Muon system. The first Triple-GEM station (GE1/1) was installed during Long Shutdown 2 (LS2, 2019-2021), and demonstrators of the second station (GE2/1) were installed in the Winter 2023-24. A new six-layer station (ME0) is scheduled for installation during the third Long Shutdown 3 (LS3, 2026-2028), and is currently in the production phase. GE1/1 and GE2/1 projects aim to reduce the p_T threshold by combining GEM and Cathode Strip Chamber (CSC) hits in the forward muon system at twice the LHC's design luminosity. In Run-3, started in 2022, the commissioning of the GE1/1 station and GE2/1 demonstrators has nearly been complete: most chambers operate stably with an efficiency exceeding 95%. Insights gained from the first CMS GEM station has improved detector and electronics design for future stations. Here we provide a general overview of the status of the three CMS GEM stations, the lessons learned from the production of GE1/1 and GE2/1, the subsequent design improvements, the start of ME0 construction, and the ongoing R&D for ME0 station.