Performance of triple GEM tracking detectors in the COMPASS experiment

COMPASS is a high-luminosity fixed target experiment at CERNʹs SPS, which has been taking data with a 160 GeV / c muon beam since 2001. The tracking of charged particles in the near-beam area is achieved by a set of twenty novel large-area micropattern gas detectors based on the Gas Electron Multiplier (GEM). Owing to a two-dimensional readout of signals, each of these detectors delivers two track projections. Distributed over a distance of 30 m throughout the spectrometer, the GEM detectors constitute the backbone of the small-area tracking system of COMPASS. The performance of these detectors in the high intensity muon beam with particle rates up to 25 kHz / mm 2 is investigated.