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 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 being stopped. Muons therefore pass easily through the inner layers of a detector, which is why muon chambers – tracking devices specialised in detecting muons – usually make up the outermost layer of a detector.
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, thus measuring their full energy. Calorimeters have to perform two different tasks at the same time – stopping particles and measuring energy loss – so they usually consist of layers of different materials: a “passive” or “absorbing” high-density material – for example, lead – interleaved with an “active” medium such as plastic scintillators 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.
The ATLAS experiment’s liquid argon calorimeter is 6.4 metres long and operates at -183ºCIn addition to measuring a particle's momentum in tracking devices and its energy in calorimeters, physicists have further methods of narrowing down its identity. These methods all rely on measuring a particle's velocity, since this, in combination with the momentum measured in the tracking devices, helps to calculate a particle's mass and therefore its identity.
Velocity can be measured using several methods. The simplest is to measure how much time it takes for a particle to travel a certain distance, using precise time-of-flight detectors. Another method looks at how much a particle ionises the matter that it passes through, as this is velocity-dependent and can be measured by tracking devices.
If a charged particle travels faster than light through a given medium, it emits Cherenkov radiation at an angle that depends on its velocity. Alternatively, when a particle crosses the boundary between two electrical insulators with different resistances to electric currents, it emits transition radiation, the energy of which depends on the particle's velocity.
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.
The job of a particle detector is to record and visualise the explosions of particles that result from the collisions at accelerators. The information obtained on a particle's speed, mass, and electric charge help physicists to work out the identity of the particle.
The work particle physicists do to identify a particle that has passed through a detector is similar to the way someone would study the tracks of footprints left by animals in mud or snow. In animal prints, factors such as the size and shape of the marks, length of stride, overall pattern, direction and depth of prints, can reveal the type of animal that came past earlier. Particles leave tell-tale signs in detectors in a similar manner for physicists to decipher.
Modern particle physics apparatus consists of layers of sub-detectors, each specialising in a particular type of particle or property. There are 3 main types of sub-detector:
To help identify the particles produced in the collisions, the detector usually includes a magnetic field. A particle normally travels in a straight line, but in the presence of a magnetic field, its path is bent into a curve. From the curvature of the path, physicists can calculate the momentum of the particle which helps in identifying its type. Particles with very high momentum travel in almost straight lines, whereas those with low momentum move forward in tight spirals.
Tracking devices reveal the paths of electrically charged particles through the trails they leave behind. There are similar every-day effects: high-flying airplanes seem invisible, but in certain conditions you can see the trails they make. In a similar way, when particles pass through suitable substances the interaction of the passing particle with the atoms of the substance itself can be revealed.
Most modern tracking devices do not make the tracks of particles directly visible. Instead, they produce tiny electrical signals that can be recorded as computer data. A computer program then reconstructs the patterns of tracks recorded by the detector, and displays them on a screen.
They can record the curvature of a particle's track (made in the presence of a magnetic field), from which the momentum of a particle may be calculated. This is useful for identifying the particle.
Muon chambers are tracking devices used to detect muons. These particles interact very little with matter and can travel long distances through metres of dense material. Like a ghost walking through a wall, muons can pass through successive layers of a detector. The muon chambers usually make up the outermost layer.
A calorimeter measures the energy lost by a particle that goes through it. It is usually designed to entirely stop 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 (lead for instance) interleaved with layers of ‘active’ medium such as solid lead-glass or liquid argon.
Electromagnetic calorimeters measure the energy of light particles – electrons and photons – as they interact with the electrically charged particles inside 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.
Two methods of particle identification work by detecting radiation emitted by charged particles: