The CMS experiment is 21 m long, 15 m wide and 15 m high, and sits in a cavern that could contain all the residents of Geneva; albeit not comfortably.

The detector is like a giant filter, where each layer is designed to stop, track or measure a different type of particle emerging from proton-proton and heavy ion collisions. Finding the energy and momentum of a particle gives clues to its identity, and particular patterns of particles or “signatures” are indications of new and exciting physics

The detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable, cooled to -268.5^oC, that generates a magnetic field of 4 Tesla, about 100,000 times that of the Earth.

Particles emerging from collisions first meet a tracker, made entirely of silicon, that charts their positions as they move through the detector, allowing us to measure their momentum. Outside the tracker are calorimeters that measure the energy of particles. In measuring the momentum, the tracker should interfere with the particles as little as possible, whereas the calorimeters are specifically designed to stop the particles in their tracks.

The Electromagnetic Calorimeter (ECAL) - made of lead tungstate, a very dense material that produces light when hit – measures the energy of photons and electrons whereas the Hadron Calorimeter (HCAL) is designed principally to detect any particle made up of quarks (the basic building blocks of protons and neutrons). The size of the magnet allows the tracker and calorimeters to be placed inside its coil, resulting in an overall compact detector.

As the name indicates, CMS is also designed to measure muons. The outer part of the detector, the iron magnet “return yoke”, confines the magnetic field and stops all remaining particles except for muons and neutrinos. The muon tracks are measured by four layers of muon detectors that are interleaved with the iron yoke. The neutrinos escape from CMS undetected, although their presence can be indirectly inferred from the “missing transverse energy” in the event.

Within the LHC, bunches of particles collide up to 40 million times per second, so a “trigger” system that saves only potentially interesting events is essential. This reduces the number recorded from one billion to around 100 per second.