Expanding the CMS Endcap Muon System: design, development and performance of the ME0 upgrade project

In the High Luminosity era of the Large Hadron Collider (HL-LHC) the delivered instantaneous luminosities will reach up to 5 ? 10³⁴ cm^-2s^-1, five times the original design value. The individual LHC experiments will require upgrades to be able to sustain this increase in luminosity and maintain their performance. The Compact Muon Solenoid (CMS) has been undergoing such upgrades to the many subsystems. The Muon System has developed a plan to use Gas Electron Multiplier (GEM) technology in the high rate endcap regions. The GEM upgrade plans include 3 stations, GE1/1, GE2/1, and ME0. The ME0 upgrade is designed to cover the most-forward areas of the CMS detector, 2.0 < |η| < 2.8, including a previously undetected area 2.4 < |η| < 2.8. With the plan to fully install the entire GEM system to CMS by 2027 testing and development is underway. An ME0 test stack of multiple GEM chambers has been assembled at CERN as part of this development. We report the status of the ME0 test stack including the design, using multiple layers for object reconstruction, and required future work.