Hermetic detector
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;[1] 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 (
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.
These machines have a hermetic construction because they are general-purpose detectors, meaning that they are able to study a wide range of phenomena in high-energy physics.
From the inside out, the first is a tracker, which measures the momentum of charged particles as they curve in a magnetic field.
The detector's magnetic field causes the particle to rotate by accelerating it in a direction perpendicular to its motion via the Lorentz force.
The tracking system plots the helix traced by such a charged particle as it travels through a magnetic field by localizing it in space in finely-segmented layers of detecting material, usually silicon.
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 the strong nuclear force.
Of all the known stable particles, only muons and neutrinos pass through the calorimeter without losing most or all of their energy.
Neutrinos cannot be directly observed at collider experiments owing to their extremely small interaction cross section with hadronic matter (such as the detector is made of), and their existence must be inferred from the so-called "missing" (transverse) energy which is computed once all other particles in the event are accounted for.
By contrast, a photon is neutral and interacts electromagnetically, so it deposits its energy in the calorimeter without leaving a track.
