In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its entirety, requiring total containment of the particle shower, or it may be sampled. Typically, calorimeters are segmented transversely to provide information about the direction of the particle or particles, as well as the energy deposited, and longitudinal segmentation can provide information about the identity of the particle based on the shape of the shower as it develops. Calorimetry design is an active area of research in particle physics.
An electromagnetic calorimeter is one specifically designed to measure the energy of particles that interact primarily via the electromagnetic interaction, while a hadronic calorimeter is one designed to measure particles that interact via the strong nuclear force. The response of a calorimeter can be described in terms of the e/h ratio. This is the measure of how well a calorimeter responds to leptons or photons versus hadrons. Ideally one would want a ratio e/h~1, this condition is called compensation.
Homogeneous versus sampling
Either of the above types can be made as a sampling calorimeter, in which the material that produces the particle shower is distinct from the material that measures the deposited energy. Typically the two materials alternate. One advantage of this is that each material can be well-suited to its task; for example, a very dense material can be used to produce a shower that evolves quickly in a limited space, even if the material is unsuitable for measuring the energy deposited by the shower. A disadvantage is that some of the energy is deposited in the wrong material and is not measured; thus the total shower energy must be estimated. A homogeneous calorimeter is one in which the entire volume is sensitive and contributes a signal.
Most particle physics experiments use some form of calorimetry. Often it is the most practical way to detect and measure neutral particles from an interaction. In addition, calorimeters are necessary for calculating "missing energy" which can be attributed to particles that rarely interact with matter and escape the detector, such as neutrinos. In most experiments the calorimeter works in conjunction with other components like a central tracker and a muon detector. All the detector components work together to achieve the objective of reconstructing a physics event.
