How a detector works

Just as hunters can identify animals from tracks in mud or snow, physicists identify subatomic particles from the traces they leave in detectors

Accelerators at CERN boost particles to high energies before they are made to collide inside detectors. The detectors gather clues about the particles – including their speed, mass and charge – from which physicists can work out a particle's identity. The process requires accelerators, powerful electromagnets, and layer upon layer of complex subdetectors.

Particles produced in collisions normally travel in straight lines, but in the presence of a magnetic field their paths become curved. Electromagnets around particle detectors generate magnetic fields to exploit this effect. Physicists can calculate the momentum of a particle – a clue to its identity – from the curvature of its path: particles with high momentum travel in almost straight lines, whereas those with very low momentum move forward in tight spirals inside the detector.

Modern particle detectors consist of layers of subdetectors, each designed to look for particular properties, or specific types of particle. Tracking devices reveal the path of a particle; calorimeters stop, absorb and measure a particle’s energy; and particle-identification detectors use a range of techniques to pin down a particle's identity.

Tracking devices

Tracking devices reveal the paths of electrically charged particles as they pass through and interact with suitable substances. 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 is stopped. For this reason, muon chambers – tracking devices specialized for detecting muons – usually make up the outermost layer of a detector.

Calorimeters

A calorimeter measures the energy a particle loses as it passes through. It is usually designed to stop entirely or “absorb” most of the particles coming from a collision, forcing them to deposit all of their energy within the detector. Calorimeters typically consist of layers of “passive” or “absorbing” high-density material – for example, lead – interleaved with layers of an “active” medium such as solid lead-glass or liquid argon.

Electromagnetic calorimeters measure the energy of electrons and photons as they interact with the electrically charged particles in matter. Hadronic calorimeters sample the energy of hadrons (particles containing quarks, such as protons and neutrons) as they interact with atomic nuclei. Calorimeters can stop most known particles except muons and neutrinos.

Particle-identification detectors

Once a particle has passed through the tracking devices and the calorimeters, physicists have two further methods of narrowing down its identity. Both methods work by detecting radiation emitted by charged particles.

When a charged particle travels faster than light does through a given medium, it emits Cherenkov radiation at an angle that depends on its velocity. The particle's velocity can be calculated from this angle. Velocity can then be combined with a measure of the particle's momentum to determine its mass, and therefore its identity.

When a fast charged particle crosses the boundary between two electrical insulators with different resistances to electric currents, it emits transition radiation. The phenomenon is related to the energy of the particle and so can distinguish different particle types.

Collating all these clues from different parts of the detector, physicists build up a snapshot of what was in the detector at the moment of a collision. The next step is to scour the collisions for unusual particles, or for results that do not fit current theories.

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