Triple GEM detectors for the Phase-2 upgrade of the CMS experiment at the LHC

Abstract The High-Luminosity LHC (HL-LHC) will increase proton-proton collision rates to 5–7.5 times the nominal LHC luminosity, with an expected number of 140–200 pp-interactions per bunch crossing (Pile-up or PU). To maintain the performance of muon triggering and reconstruction under high background, the forward part of the Muon spectrometer of the CMS experiment will be upgraded with Gas Electron Multiplier (GEM) and improved Resistive Plate Chamber (iRPC) detectors. A first GEM station (GE1/1) was installed during Long Shutdown 2 (LS2, 2019–2021) while second and third stations (GE2/1 and ME0) will be installed in the third Long Shutdown (LS3, 2026–2028) and during the Run 4 technical stops. GE1/1, considered an early Phase-2 upgrade, will reduce the p T threshold by combining GEM and Cathode Strip Chamber (CSC) hits in the forward muon system at twice the LHC design luminosity (2 × 10 34 /cm 2 s, 50 PU). The commissioning of the GE1/1 detector has been completed during Run 3 in 2022 and 2023. Most chambers are operating stably with an efficiency higher than 95%. Demonstration of the combined CSC-GEM trigger will take place in 2024. The first GE2/1 detectors were installed in the LHC 2023–24 technical stop and the ME0 detector is entering the production phase in 2024. This presentation discusses the commissioning and early performance of GE1/1, together with the lessons learnt which were used to improve the detector and electronics design for GE2/1 and ME0. We also present the R&D performed to optimize the ME0 detector for its high-rate environment, together with the performance measurements of the first stack of ME0 detectors for standalone muon segment reconstruction.