New gas electron-multiplier detectors for the endcap muon system of the CMS experiment at the high-luminosity LHC design and prototype performance

The high luminosity LHC will require new detectors in the CMS endcap muon system to suppress the trigger rate of background events, to maintain high trigger efficiency for low transverse momentum muons, to enhance the robustness of muon detection in the high-flux environment of the endcap, and to extend the geometrical acceptance. We report on the design and recent progress towards implementing a new system of large-area, triple-foil gas electron-multiplier (GEM) detectors that will be installed in the first three of five muon detector stations in each endcap, the first station being closest to the interaction point. The first station will extend the geometric acceptance in pseudo-rapidity to eta lt 3.0 from the current limit of eta lt 2.4. The second and third stations will enhance the performance in the range 1.6 lt eta lt 2.4. We describe the design of the chambers and readout electronics and report on the performance of prototype systems in tests with cosmic ray muons, high-energy particlebeams, and under high intensity gamma irradiation. We describe plans to install a prototype system in the CMS experiment in 2017, with subsequent operation during the current Run 2 of the LHC.