The detectors sit within a magnetic field that bends the tracks of charged particles. The momentum of a particle – which gives a clue to its identity – can be calculated from the curvature of its path, as particles with high momentum travel in almost straight lines, whereas those with very low momentum move forward in tight spirals inside the detector. In addition, the magnetic field is useful in the identification of the charge of the produced particles.
The experiment is divided into a number of components with each component giving information about a specific set of particle properties. These components are stacked in layers and the particles go through them sequentially. From the collision point outwards, first there are the tracking systems, then the electromagnetic (EM) and hadronic calorimeters and finally the muon system. The tracking systems reveal the path of a particle; calorimeters stop, absorb, and measure its energy; and particle-identification detectors use a range of techniques to pin down a particle's identity.
Tracking devices reveal the paths of electrically charged particles as they pass through and interact with the device's material. Most tracking devices do not make particle tracks directly visible, but record tiny electrical signals that particles trigger as they move through the device. A computer program then reconstructs the recorded patterns of tracks. One type of particle, the muon, interacts very little with matter – it can travel through metres of dense material before it stops. For this reason, muon chambers – tracking devices specialized in muon detection – usually make up the outermost layer of a detector.
Calorimeters are either electromagnetic or hadronic. The former detect particles such as electrons and photons, while the latter pick up protons and neutrons. Particles entering a calorimeter are absorbed and a particle shower is created by cascades of interactions. It is the energy deposited in the calorimeter from these interactions that is measured.
Particle identification is crucial for heavy-ion physics. To narrow down the particle's identity a number of different methods are used. Charged particles are unambiguously identified if their mass and charge are determined. The mass can be deduced from measurements of the momentum and of the velocity. Momentum and the sign of the charge are obtained by measuring the curvature of the particle’s track in a magnetic field. To obtain the particle velocity there are four methods based on measurements of time-of-flight (TOF) and ionization, and on the detection of transition radiation (TR) and Cherenkov radiation. Each method works well in different momentum ranges or for specific types of particle.
By collating clues from all the different parts of the detector, we can create a snapshot of what happened in the detector at the moment of a collision.