CERN - European Organization for Nuclear Research

The CMS detector is built around a huge solenoid magnet: a cylindrical superconducting coil that generates a magnetic field of 4 teslas, about 100 000 times that of the Earth. Measuring 6 m in diameter and 12 m in length, it is the largest magnet of its kind ever built. The experiment will study proton and heavy-ion collisions. Each layer of the detector will reveal a different facet of what happens during these collisions.

As the magnet is exceptionally large, it was possible to place the majority of the sub-detectors inside it. A silicon tracker located closest to the collision point will reveal the tracks of the particles and measure their momentum. The next layers are the crystal electromagnetic calorimeter, which will measure the energy of the photons, electrons and positrons, followed by the hadron calorimeter, which will measure the energy of particles such as pions, protons and neutrons.
A huge red-painted iron yoke outside the magnet returns the magnetic field so that the muon chambers are also within the field. The 11 enormous 12-sided sections that form the yoke also provide a mechanical frame for the whole detector.

The LHC will produce up to 600 million collisions per second. This myriad of events will be filtered by an electronic trigger system which will select only the most interesting events, reducing them to 100 per second. This nonetheless represents a volume of data equivalent to 10 000 copies of the Encyclopaedia Britannica every second!