Tracking detectors are used to measure the positions of a particle. The particles ionise the medium through which they pass, which can then be translated into a position in space. The central tracking detector (CTD) is used with magnetic fields to bend the particles.

F = BQv, equation 1

F = mv^2 / r, equation 2

where:

• F = force
• B = magnetic field strength
• v = velocity
• r = radius

So the momentum of the particle can be measured by taking the radius of curvature of the track using equations 1 and 2 above. Only one component of momentum will be bent by the magnetic field, which is its perpendicular component. The component in the same direction as the magnetic field will not be affected. The charge of the particle can be explained by the direction of the curvature. Take for instance positively charged particles to bend inwards and negatively charged particles to therefore be bent outwards.

1. Vertex detector, to reveal decays in beam pipe
2. Tracking detector
3. Magnet coil, to bend tracks for momentum measurement
4. Time-of-flight detector, for velocity measurement
5. Electromagnetic calorimeter, to detect eletrons and photons
6. Hadron calorimeter, to detect protons, pion etc..
7. Muon detectors
8. Forward detectors, for particles close to beam pipe