Performance of a Large-Area Triple-GEM Detector in a Particle Beam

A multi-institutional collaboration is investigating the possibility of enhancing muon tracking and triggering capabilities in the small-angle region $1.6 < \rm{abs}(\eta) < 2.1$ of the CMS experiment at the LHC by instrumenting the end-cap muon system with large-area gas electron multiplier (GEM) detectors. A first trapezoidal prototype triple-GEM detector of size 1 m by 0.5 m was built and operated successfully in a test beam at CERN in October 2010. Front-end readout boards utilizing the ``VFAT" chip are mounted in a regular array directly on the chambers. High voltage is provided by a compact divider board implemented with surface mount components. A tracker equipped with small GEM detectors was used to precisely measure the hit position in the large-area detector. A spatial resolution of 0.29 mm was measured in a region with average strip pitch of 1.1 mm, and $\sim$98\% hit efficiency was achieved at full operating voltage. An additional prototype chamber is constructed and will undergo beam tests this summer with operation in a 3T magnetic field.