The LHC experiments

The LHC experiments

The six experiments at the LHC are all run by international collaborations, bringing together scientists from institutes all over the world. Each experiment is distinct, characterised by its unique particle detector.
The two large experiments, ATLAS and CMS, are based on general-purpose detectors to analyse the myriad of particles produced by the collisions in the accelerator. They are designed to investigate the largest range of physics possible. Having two independently designed detectors is vital for cross-confirmation of any new discoveries made.
Two medium-size experiments, ALICE and LHCb, have specialised detectors for analysing the LHC collisions in relation to specific phenomena.
Two further experiments, TOTEM and LHCf, are much smaller in size. They are designed to focus on "forward particles" (protons or heavy ions). These are particles that just brush past each other as the beams collide, rather than meeting head-on.
The ATLAS, CMS, ALICE and LHCb detectors are installed in four huge underground caverns located around the ring of the LHC. The detectors used by the TOTEM experiment are positioned near the CMS detector, whereas those used by LHCf are near the ATLAS detector.

ATLAS

ATLAS is one of two general-purpose detectors at the LHC. It will investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter. ATLAS will record sets of measurements on the particles created in collisions - their paths, energies, and their identities.
This is accomplished in ATLAS through six different detecting subsystems that identify particles and measure their momentum and energy.
Another vital element of ATLAS is the huge magnet system that bends the paths of charged particles for momentum measurement.
The interactions in the ATLAS detectors will create an enormous dataflow. To digest these data, ATLAS needs a very advanced trigger and data acquisition system, and a large computing system.
More than 2900 scientists from 172 institutes in 37 countries work on the ATLAS experiment (December 2009).

ATLAS detector

• Size: 46 m long, 25 m high and 25 m wide. The ATLAS detector is the largest volume particle detector ever constructed.
• Weight: 7000 tonnes
• Design: barrel plus end caps
• Location: Meyrin, Switzerland.

Related links

• The ATLAS website
•  

  How a detector works

  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:
  

  • Tracking device – detects and reveals the path of a particle
  • Calorimeter – stops, absorbs and measures the energy of a particle
  • Particle identification detector – identifies the type of particle using various techniques

  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

  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.
  

  Calorimeters

  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.
  

  Particle identification detectors

  Two methods of particle identification work by detecting radiation emitted by charged particles:
  

  • Cherenkov radiation: this is light emitted when a charged particle travels faster than the speed of light through a given medium. The light is given off at a specific angle according to the velocity of the particle. Combined with a measurement of the momentum of the particle the velocity can be used to determine the mass and hence to identify the particle.
  • Transition radiation: this radiation is produced by a fast charged particle as it crosses the boundary between two electrical insulators with different resistances to electric currents. The phenomenon is related to the energy of a particle and distinguishes different particle types.
  •  

    How a detector works

    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:
    

    • Tracking device – detects and reveals the path of a particle
    • Calorimeter – stops, absorbs and measures the energy of a particle
    • Particle identification detector – identifies the type of particle using various techniques

    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

    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.
    

    Calorimeters

    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.
    

    Particle identification detectors

    Two methods of particle identification work by detecting radiation emitted by charged particles:
    

    • Cherenkov radiation: this is light emitted when a charged particle travels faster than the speed of light through a given medium. The light is given off at a specific angle according to the velocity of the particle. Combined with a measurement of the momentum of the particle the velocity can be used to determine the mass and hence to identify the particle.
    • Transition radiation: this radiation is produced by a fast charged particle as it crosses the boundary between two electrical insulators with different resistances to electric currents. The phenomenon is related to the energy of a particle and distinguishes different particle types.
    
    

    Missing Higgs

    A major breakthrough in particle physics came in the 1970s when physicists realized that there are very close ties between two of the four fundamental forces – namely, the weak force and the electromagnetic force. The two forces can be described within the same theory, which forms the basis of the Standard Model. This ‘unification’ implies that electricity, magnetism, light and some types of radioactivity are all manifestations of a single underlying force called, unsurprisingly, the electroweak force. But in order for this unification to work mathematically, it requires that the force-carrying particles have no mass. We know from experiments that this is not true, so physicists Peter Higgs, Robert Brout and François Englert came up with a solution to solve this conundrum.
    They suggested that all particles had no mass just after the Big Bang. As the Universe cooled and the temperature fell below a critical value, an invisible force field called the ‘Higgs field’ was formed together with the associated ‘Higgs boson’. The field prevails throughout the cosmos: any particles that interact with it are given a mass via the Higgs boson. The more they interact, the heavier they become, whereas particles that never interact are left with no mass at all.
    This idea provided a satisfactory solution and fitted well with established theories and phenomena. The problem is that no one has ever observed the Higgs boson in an experiment to confirm the theory. Finding this particle would give an insight into why particles have certain mass, and help to develop subsequent physics. The technical problem is that we do not know the mass of the Higgs boson itself, which makes it more difficult to identify. Physicists have to look for it by systematically searching a range of mass within which it is predicted to exist. The yet unexplored range is accessible using the Large Hadron Collider, which will determine the existence of the Higgs boson. If it turns out that we cannot find it, this will leave the field wide open for physicists to develop a completely new theory to explain the origin of particle mass.