How we see particles
For each collision, the physicist’s goal is to count, track and characterize all the different particles that were produced in order to reconstruct the process in full. Just the track of the particle gives much useful information, especially if the detector is placed inside a magnetic field: the charge of the particle, for instance, will be obvious since particles with positive electric charge will bend one way and those with negative charge will bend the opposite way. Also the momentum of the particle (the ‘quantity of motion’, which is equal to the product of the mass and the velocity) can be determined: very high momentum particles travel in almost straight lines, low momentum particles make tight spirals.
The purpose of the large detectors installed at particle colliders is to identify the secondary particles produced in collisions, and to measure their positions in space, their charges, speed, mass and energy.
To do this, the detectors have many layers or ‘sub-detectors’ that each have a particular role in the reconstruction of collisions. A magnet system completes the design. Its function is to separate the different particles according to their charge and to allow the measurement of their momentum - a physical quantity linked to the mass and speed of the particle.
There are two important categories of subdetector:
} Tracking devices reveal the tracks of electrically charged particles through the trails they leave by ionizing matter. In a magnetic field they can be used to measure the curvature of a particle’s trajectory and hence the particle’s momentum. This can help in identifying the particle. Most modern tracking devices do not make the tracks directly visible. Instead, they produce electrical signals that can be recorded as computer data. A computer program reconstructs the patterns of tracks recorded.
Two specialized types of tracking devices are vertex detectors and muon chambers. Vertex detectors are located close to the interaction point (primary vertex); muon chambers are located at the outer layers of a detector assembly because muons are the only charged particles able to travel through metres of dense material.
There are two main techniques used to build tracking devices:
} Gaseous chambers, where the medium ionized is a gas and the ions or electrons are collected on electrodes usually in the form of wires or pads under strong electric fields. In drift chambers, the position of the track is found by timing how long the electrons take to reach an anode wire, measured from the moment that the charged particle passed through. This results in higher spatial resolution for wider wire separation: drift cells are typically several centimetres across, giving a spatial resolution of 50-100mm. In a time projection chamber the drift volume is much larger, up to 2m or more, and the sense wires are arranged on one end face.
} Semiconductor detectors, where the particle creates electrons and holes as it passes through a reverse-biased semiconductor, usually silicon. The devices are subdivided into strips or pixels. Typical resolution is 10mm.
} Calorimeters, devices that measure the energy of particles by stopping them and measuring the amount of energy released. There are two main types of calorimeter: electromagnetic (ECAL) and hadronic (HCAL). They use different materials depending on which type of particle they are stopping. The ECAL generally fully absorbs electrons and photons, which interact readily through the electromagnetic force. Strongly interacting particles (hadrons), such as protons and pions, may begin to lose energy in the ECAL but will be stopped in the HCAL. Muons (and neutrinos) will pass through both layers. Calorimeters provide the main way to identify neutral particles such as photons and neutrons; although they are not visible in tracking devices, they are revealed by the energy they deposit in the calorimeters.
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.
} Detectors also often have sub-detectors measuring the speed of charged particles, an essential factor for particle identification.
There are two important methods for measuring the velocity of particles:
} Cherenkov radiation: when a charged particle traverses a medium, above a certain velocity, it emits photons at a specific angle that depends on the velocity. When 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. For Cherenkov emission to occur the particle must be travelling faster than the speed of light in the medium.
} Transition radiation: when a relativistic charged particle traverses an inhomogeneous medium, in particular the boundary between materials with different electrical properties, it emits radiation more or less in proportion to its energy. This allows particle types to be distinguished from each other.