Large-size triple GEM detectors for the CMS forward muon upgrade

The CMS collaboration considers upgrading the muon forward region which is particularly affected by the high-luminosity conditions at the LHC. The proposal involves Gas Electron Multiplier (GEM) chambers, which are able to handle the extreme particle rates expected in this region along with a high spatial resolution. This allows to combine tracking and triggering capabilities, which will improve the CMS muon High Level Trigger, the muon identification and the track reconstruction. Intense R\&D has lead to the development of several GEM prototypes and associated detector electronics which were operated in several test beams. Strip cluster parameters, detection efficiency and spatial resolution for charged particles are studied with position and high voltage scans and at different inclination angles. A first GEM station is foreseen to be already installed in LHC Phase-I to operate jointly together with the existing CSC detectors in the forward region. The resulting improved transverse momentum assignment and lower trigger fake rate will reduce the needed bandwidth and therefore allow to lower the trigger threshold resulting in an increased sensitivity in channels triggering on soft muons, such as Higgs boson decaying to tau's with subsequent leptonic decays to muons. Further GEM detectors are proposed for LHC Phase-II to significantly increase the forward muon acceptance.