Hermetic detector
Encyclopedia
In particle physics
, a hermetic detector (also called a 4π detector) is a particle detector
designed to observe all possible decay products of an interaction between subatomic particle
s in a collider
by covering as large an area around the interaction point
as possible and incorporating multiple types of sub-detectors. They are typically roughly cyllindrical, with different types of detectors wrapped around each other; each detector type specializes in particular particles so that almost any particle will be detected and identified. Such detectors are called "hermetic" because they are designed to let as few particles as possible escape; the name "4π detector" comes from the fact that such detectors are designed to cover nearly all of the 4π steradian
s of solid angle
around the interaction point.
The first such detector was the Mark I
at the Stanford Linear Accelerator Center
, and the basic design has been used for all subsequent collider detectors. Prior to the building of the Mark I, it was thought that most particle decay products would have relatively low transverse momentum (i.e. momentum perpendicular to the beamline
), so that detectors could cover this area only. However, it was learned at the Mark I and subsequent experiments that most fundamental particle interactions at colliders involve very large exchanges of energy and therefore large transverse momenta are not uncommon; for this reason, large angular coverage is critical for modern particle physics.
There are three main components of a hermetic detector. From the inside out, the first is a tracker, which measures the momentum
of charged particles as they curve in a magnetic field
. Next there are one or more calorimeters, which measure the energy of most charged and neutral
particles by absorbing them in dense material, and a muon
system which measures the one type of particle that is not stopped through the calorimeters and can still be detected. Each component may have several different specialized sub-components.
traced by a charged particle that curves in a magnetic field by localizing it in space in finely-segmented layers of detecting material, usually silicon
. The degree to which the particle curves is inversely proportional to its momentum perpendicular to the beam, while the degree to which it drifts in the direction of the beam axis gives its momentum in that direction.
, and the hadronic calorimeter that can detect hadrons, which interact via the strong nuclear force. A hadronic detector is required in particular to detect heavy neutral particle
s.
s and neutrino
s pass through the calorimeter without losing most or all of their energy. Neutrinos are practically undetectable, and their existence must be inferred, but muons (which are charged) can be measured by an additional tracking system outside the calorimeters.
is charged and interacts electromagnetically, so it is tracked by the tracker and then deposits all of its energy in the (electromagnetic) calorimeter. By contrast, a photon
is neutral and interacts electromagnetically, so it deposits its energy in the calorimeter without leaving a track.
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, a hermetic detector (also called a 4π detector) is a particle detector
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
designed to observe all possible decay products of an interaction between subatomic particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
s in a collider
Collider
A collider is a type of a particle accelerator involving directed beams of particles.Colliders may either be ring accelerators or linear accelerators.-Explanation:...
by covering as large an area around the interaction point
Interaction point
In particle physics, an interaction point is the place where particles collide. One differentiates between the nominal IP, which is the design position of the IP, and the real or physics IP, which is the position where the particles actually collide...
as possible and incorporating multiple types of sub-detectors. They are typically roughly cyllindrical, with different types of detectors wrapped around each other; each detector type specializes in particular particles so that almost any particle will be detected and identified. Such detectors are called "hermetic" because they are designed to let as few particles as possible escape; the name "4π detector" comes from the fact that such detectors are designed to cover nearly all of the 4π steradian
Steradian
The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...
s of solid angle
Solid angle
The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
around the interaction point.
The first such detector was the Mark I
Mark I (detector)
The Mark I, also known as the SLAC-LBL Magnetic Detector, was a particle detector that operated at the interaction point of the SPEAR collider from 1973 to 1977. It was the first 4π detector, i.e...
at the Stanford Linear Accelerator Center
Stanford Linear Accelerator Center
The SLAC National Accelerator Laboratory, originally named Stanford Linear Accelerator Center, is a United States Department of Energy National Laboratory operated by Stanford University under the programmatic direction of the U.S...
, and the basic design has been used for all subsequent collider detectors. Prior to the building of the Mark I, it was thought that most particle decay products would have relatively low transverse momentum (i.e. momentum perpendicular to the beamline
Beamline
In particle physics, a beamline is the line in a linear accelerator along which a beam of particles travels. It may also refer to the line of travel within a bending section such as a storage ring or cyclotron, or an external beam extracted from a cyclic accelerator.In materials science, physics,...
), so that detectors could cover this area only. However, it was learned at the Mark I and subsequent experiments that most fundamental particle interactions at colliders involve very large exchanges of energy and therefore large transverse momenta are not uncommon; for this reason, large angular coverage is critical for modern particle physics.
Components
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of charged particles as they curve in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
. Next there are one or more calorimeters, which measure the energy of most charged and neutral
Neutral particle
In physics, a neutral particle is a particle with no electric charge. This is not to be confused with a real neutral particle, a neutral particle that is also identical to its own antiparticle.-Stable or long-lived neutral particles:...
particles by absorbing them in dense material, and a muon
Muon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
system which measures the one type of particle that is not stopped through the calorimeters and can still be detected. Each component may have several different specialized sub-components.
Trackers
The tracking system plots the helixHelix
A helix is a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. Examples of helixes are coil springs and the handrails of spiral staircases. A "filled-in" helix – for...
traced by a charged particle that curves in a magnetic field by localizing it in space in finely-segmented layers of detecting material, usually silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
. The degree to which the particle curves is inversely proportional to its momentum perpendicular to the beam, while the degree to which it drifts in the direction of the beam axis gives its momentum in that direction.
Calorimeters
Calorimeters slow particles down and absorb their energy into a material, allowing that energy to be measured. They are often divided into two types: the electromagnetic calorimeter that specializes in absorbing particles that interact electromagneticallyElectromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
, and the hadronic calorimeter that can detect hadrons, which interact via the strong nuclear force. A hadronic detector is required in particular to detect heavy neutral particle
Neutral particle
In physics, a neutral particle is a particle with no electric charge. This is not to be confused with a real neutral particle, a neutral particle that is also identical to its own antiparticle.-Stable or long-lived neutral particles:...
s.
Muon system
Of all the known stable particles, only muonMuon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
s and neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s pass through the calorimeter without losing most or all of their energy. Neutrinos are practically undetectable, and their existence must be inferred, but muons (which are charged) can be measured by an additional tracking system outside the calorimeters.
Particle identification
Most particles have unique combinations of signals left in each detector sub-system, allowing different particles to be identified. For example, an electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
is charged and interacts electromagnetically, so it is tracked by the tracker and then deposits all of its energy in the (electromagnetic) calorimeter. By contrast, a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
is neutral and interacts electromagnetically, so it deposits its energy in the calorimeter without leaving a track.
See also
- ATLAS experimentATLAS experimentATLAS is one of the six particle detector experiments constructed at the Large Hadron Collider , a new particle accelerator at the European Organization for Nuclear Research in Switzerland...
, for a detailed description of such a detector. - Compact Muon SolenoidCompact Muon SolenoidThe Compact Muon Solenoid experiment is one of two large general-purpose particle physics detectors built on the proton-proton Large Hadron Collider at CERN in Switzerland and France. Approximately 3,600 people from 183 scientific institutes, representing 38 countries form the CMS collaboration...
, for a well-illustrated description of another such detector.