The COMPASS experiment in building 888 has started running, after several tests made last year. It will basically investigate the structure and spectroscopy of hadrons and is scheduled to run beyond the start of the LHC.

View of the COMPASS detector from behind in building 888 in Prévessin.

Finally its time has come! There has been frantic activity in building 888, the COMPASS (Common Muon Proton Apparatus for Structure and Spectroscopy) kingdom. Everything had to be ready for May 27, when the SPS started extracting protons to feed the M2 beam line and the experiment began a full data taking year for the first time. (Last year the first data were collected after a long commissioning phase with an incomplete spectrometer.) COMPASS will focus on learning more about hadrons - particles made of quarks, including the nucleons (protons and neutrons) of ordinary matter. It will try to find out how hadrons are built up and in particular what contributes to the spin of the nucleon. A total of 220 physicists coming mainly from France, Germany, Italy, Japan and Russia are involved in this project, led by Franco Bradamante and Stephan Paul. In 98 days of beam, they expect to obtain the first results on the polarisation of gluons in the nucleon.
COMPASS is 60 metres long, 10 metres high and resembles an accordion with a structure that is repeated twice. It is designed as a double forward spectrometer, each section equipped with Ring Imaging Cherenkov (RICH) detectors, electromagnetic and hadronic calorimeters and muon filters for particle identification. Large spectrometer magnets and a variety of tracking detectors determine the particle momenta. The first stage of the installation is complete. The second stage, foreseen for the future, will also include a second RICH detector and the final electromagnetic calorimeter. 'We look forward to getting these pieces in the future, so we can exploit the full physics programme of our experiment. For the moment, we can already get interesting data for the first part of the programme with the detectors we have', explains Gerhard Mallot, technical coordinator for COMPASS.

The first part of the COMPASS detector, with the Polarized Target.

The first section of the apparatus analyses particles with large angles, while the second one focuses on smaller angle, faster particles. The particles take the following journey. A beam pulse of 10¹³ protons, yielding a muon pulse of 2 x 10⁸, is taken by the beam line from the SPS to the polarised target, where nucleons with their magnetic moments oriented at a temperature of 50 mK are awaiting the collisions. Then the spectrometer tracks the emerging particles before they arrive in the RICH detector. The RICH is a Ring Imaging Cherenkov which identifies particles coming from the collisions in the COMPASS target; in particular, it distinguishes pions and kaons with momenta up to 60 GeV/c. Particles travel though the C₄F₁₀ gas contained in the RICH vessel with a speed faster than that of light, causing the Cerenkov effect, a phenomenon analogous to the sound of planes when they go through the sound barrier. It's a 'bang', but in this case it's made of rings of light, which are focused by two spherical mirrors - made up of 120 hexagonal and pentagonal reflecting surfaces - onto two sets of detectors. These detectors are equipped with caesium iodide photocathodes, a new technique developed at CERN in the RD26 project, which converts the light into an electrical pulse. Afterwards, hadrons deposit all their energy in the hadron calorimeter and stop, so that only muons pass through, finally reaching the muon detectors beyond the absorber wall.
There are a many features that make COMPASS a unique experiment. It is the first time that such a big experiment uses novel detector techniques, such as the Micro Mesh Gaseous Structure (Micromegas), developed by Nobel Prize Winner Georges Charpak, and the Gas Electron Multiplier (GEM) developed by Fabio Sauli at CERN. The Micromegas cover the central part of the first spectrometer, while the inner region of the second spectrometer is equipped with GEM detectors. Both these gaseous detectors amplify the electrons knocked out of a gas by charged particles as they pass through. For the Micromegas this happens in a tiny gap of 0.1 mm between a metallic mesh and the anode. The GEM, on the other hand, is a thin sheet of plastic coated with metal on both sides and chemically pierced by a regular array of holes a fraction of a millimetre across, and apart, in which the amplification takes place. Large-area tracking is provided by so-called straw detectors and high flux drift chambers working with the same physical principle. Scintillating fibres and scintillator hodoscopes provide highly precise time information and the incoming beam is measured by silicon detectors.
The COMPASS detector electronics remember the path of the particles and after 500 ns a decision is made on whether an interaction is interesting enough to keep a record of it. Despite the careful selection of events, the remaining data volume of 200 to 300 TByte/year will still be enormous; the handling and analysis of such data volumes constitutes a real technical challenge. However, the powerful COMPASS Computing Farm with 200 CPUs is up to this challenge and will help to reveal the interior of matter.