Charged particle detection performance of Gas Electron Multiplier (GEM) detectors for the upgrade of CMS endcap muon system at the CERN LHC

The Compact Muon Solenoid (CMS) detector is one of the two general-purpose detectors at the CERN LHC. LHC will provide exceptional high instantaneous and integrated luminosity after second long shutdown. The forward region |η| ≥ 1:5 of CMS detector will face extremely high particle rates in tens of kHz/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and hence it will affect the momentum resolution, efficiency and longevity of the muon detectors. Here, η is pseudorapidity defined as η = −ln(tan(θ/2)), where θ is the polar angle measured from z-axis. To overcome these issues the CMSGEM collaboration has proposed to install new large size rate capable Triple Gas Electron Multiplier (GEM) detectors in the forward region of CMS muon system. The first set of Triple GEM detectors will be installed in the GE1/1 region (1:6 < |η| < 2.2) of the muon endcap during the long shutdown 2 (LS2) of the LHC. Towards this goal, full size CMS Triple GEM detectors have been fabricated and tested at the CERN SPS, H2 and H4 test beam facility. The GEM detectors were operated with two gas mixtures: Ar/CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (70/30) and Ar/CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /CF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> (45/15/40). In 2014, good quality data was collected during test beam campaigns. In this paper, the performance of the detectors is summarized based on their tracking efficiency and time resolution.