- Hermetic detector
In
particle physics , a hermetic detector (also called a 4π detector) is aparticle detector designed to observe all possible decay products of an interaction betweensubatomic particle s in acollider by covering as large an area around theinteraction 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 "" 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 4πsteradian s ofsolid 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 thebeamline ), 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
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 amagnetic 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 amuon 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
helix traced by a charged particle that curves in a magnetic field by localizing it in space in finely-segmented layers of detecting material, usuallysilicon . 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 electromagnetically, and the hadronic calorimeter that can detect
hadrons , which interact via thestrong nuclear force . A hadronic detector is required in particular to detect heavyneutral particle s.Muon system
Of all the known stable particles, only
muon s andneutrino s pass through the calorimeter without losing most or all of their energy. Neutrinos are undetectable, and their existence must be inferred, butmuons (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
electron 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, aphoton is neutral and interacts electromagnetically, so it deposits its energy in the calorimeter without leaving a track.See also
*
ATLAS experiment , for a detailed description of such a detector.
*Compact Muon Solenoid , for a well-illustrated description of another such detector.
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