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 particles 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 cylindrical, with different types of detectors wrapped around each other in concentric layers; 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 constructed so as the motion of particles are ceased at the boundaries of the chamber without any moving beyond due to the seals; the name "4π detector" comes from the fact that such detectors are designed to cover nearly all of the 4π steradians of solid angle around the interaction point; in terms of the standard coordinate system used in collider physics, this is equivalent to coverage of the entire range of azimuthal angle () and pseudorapidity (). In practice, particles with pseudorapidity above a certain threshold cannot be measured since they are too nearly parallel to the beamline and can thus pass through the detector. This limit on the pseudorapidity ranges which can be observed forms part of the acceptance of the detector (i.e. the range of phase space which it is able to observe); broadly speaking, the main design objective of a hermetic detector is to maximise acceptance, i.e. to ensure that the detector is able to measure as large a phase space region as possible. 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. More recent hermetic detectors include the CDF and DØ detectors at Fermilab's Tevatron accelerator, as well as the ATLAS and CMS detectors at CERN's LHC. These machines have a hermetic construction because they are general-purpose