  For its physics program with a high-intensity hadron beam of up to 2e7
particles/s, the COMPASS experiment at CERN requires tracking of charged
particles scattered by very small angles with respect to the incident beam
direction. While good resolution in time and space is mandatory, the challenge
is imposed by the high beam intensity, requiring radiation-hard detectors which
add very little material to the beam path in order to minimize secondary
interactions.
  To this end, a set of triple-GEM detectors with a hybrid readout structure
consisting of pixels in the beam region and 2-D strips in the periphery was
designed and built. Successful prototype tests proved the performance of this
new detector type, showing both extraordinary high rate capability and
detection efficiency. The amplitude information allowed to achieve spatial
resolutions about a factor of 10 smaller than the pitch and a time resolution
close to the theoretical limit imposed by the layout.
  The PixelGEM central tracking system consisting of five detectors, slightly
improved with respect to the prototype, was completely installed in the COMPASS
spectrometer in spring 2008.
