The Compass Experiment at CERN

The COMPASS experiment makes use of the CERN SPS high-intensity muon and hadron beams for the investigation of the nucleonspin structure and the spectroscopy of hadrons. One or more outgoing particles are detected in coincidence with the incoming muon orhadron. A large polarised target inside a superconducting solenoid is used for the measurements with the muon beam. Outgoing particlesare detected by a two-stage, large angle and large momentum range spectrometer. The setup is built using several types of trackingdetectors, according to the expected incident rate, required space resolution and the solid angle to be covered. Particle identification isachieved using a RICH counter and both hadron and electromagnetic calorimeters. The setup has been successfully operated from 2002onwards using a muon beam. Data with a hadron beam were also collected in 2004. This article describes the main features andperformances of the spectrometer in 2004; a short summary of the 2006 upgrade is also given