This report describes both the technical design and the expected performance of the Phase-II upgrade, using Gas Electron Multiplier (GEM) detectors, of the first endcap station of the CMS muon system. The upgrade is targeted for the second long shutdown of the CERN LHC and is designed to improve the muon trigger and tracking performance at high luminosity. The GEM detectors will add redundancy to the muon system in the 1.6 < |η| < 2.2 pseudorapidity region, where the amount of detection layers is lowest while the background rates are highest and the bending of the muon trajectories due to the CMS magnetic field is small. GEM detectors have been identified as a suitable technology to operate in the high radiation environment present in that region. The first muon endcap station will be instrumented with a double layer of triple-GEM chambers in the 1.6 < |η| < 2.2 region. The detector front-end electronics uses the custom designed VFAT3 chip to provide both fast input for the level-1 muon trigger and full granularity information for offline muon reconstruction. This document describes the design of detectors, electronics, and services. The expected performance of the upgraded muon system is discussed in the context of several benchmark physics channels. The document also presents the plan - including the project schedule, cost, and organization - for the detector construction, testing, and integration into the CMS detector.
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