particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, a calorimeter is an experimental apparatus that measures the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

particles In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass. They vary greatly in size or quantity, from s ...

. Most particles enter the

calorimeter A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimete ...

and initiate a

particle shower In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these t ...

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 In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in ''state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...

design is an active area of research in particle physics.

Types of calorimeters

Electromagnetic versus hadronic

An electromagnetic calorimeter is one specifically designed to measure the energy of particles that interact primarily via the

electromagnetic interaction In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

, while a hadronic calorimeter is one designed to measure particles that interact via the

strong nuclear force The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called the ...

. (See types of particle showers for the differences between the two.) 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.Particle Physics Booklet 2006 pg 272

Calorimeters in high energy physics experiments

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.

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