How does a detector work?

Particle detectors are very simple in principle, but extremely complex in practice.

The detectors at the LHC are built around the collision points where the particle beams meet head-on and they are designed to track the motion and measure the energy and charge of the new particles thrown out in all directions from the collisions. The LHC detectors are very large, for example ATLAS is the size of a 5 storey building. Their great size is necessary firstly, to trap high energy particles travelling near the speed of light and secondly, to allow the tracks of charged particles to be detectably curved by the detector magnets.

Detectors are typically made up of layers, like an onion, with each layer designed to detect different properties of the particles as they travel through the detector. The layers nearest to the collision point are designed to very precisely track the movement of particles, especially the short-lived particles that are both the most difficult to detect and the most interesting to the researchers.

Subsequent layers track the movement, and also slow down and stop, longer-lived and more energetic particles. As these particles are slowed down they release energy that is measured by these layers (the calorimeters).

Detectors usually include a powerful magnet; this affects the motion of charged particles produced in collisions and from the extent of its effect researchers can measure the charge and momentum of particles.