Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil (nothing). A literal translation is "to make into nothing".

In physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

, the word is used to denote the process that occurs when a subatomic particle

Subatomic particle

In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...

 collides with its respective antiparticle

Antiparticle

Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...

. Since energy and momentum must be conserved, the particles are not actually made into nothing, but rather into new particles. Antiparticles have exactly opposite additive quantum number

Quantum number

Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...

s from particles, so the sums of all quantum numbers of the original pair are zero. Hence, any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy

Conservation of energy

The nineteenth century law of conservation of energy is a law of physics. It states that the total amount of energy in an isolated system remains constant over time. The total energy is said to be conserved over time...

 and conservation of momentum are obeyed. When a particle and its antiparticle collide, their energy is converted into a force carrier particle, such as a gluon, W/Z force carrier particle, or a photon. These particles are afterwards transformed into other particles.

During a low-energy annihilation, photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

 production is favored, since these particles have no mass. However, high-energy particle colliders produce annihilations where a wide variety of exotic heavy particles are created.

Examples of annihilation

This is an example of renormalization

Renormalization

In quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities....

 in quantum field theory

Quantum field theory

Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

— the field theory being necessary because the number of particles changes from one to two and back again.

Electron–positron annihilation

 +  →  + 

When a low-energy electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 annihilates a low-energy positron

Positron

The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...

 (antielectron), they can only produce two or more gamma ray

Gamma ray

Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, since the electron and positron do not carry enough mass-energy to produce heavier particles and conservation of energy and linear momentum forbid the creation of only one photon.When an electron and a positron collide to annihilate and create gamma rays, energy is given off. Both particles have a rest energy of 0.511 MeV or million electron volts. When the mass of the two particles are converted entirely into energy, this rest energy is what is given off. The energy is given off in the form of the aforementioned gamma rays. Each of the gamma rays have an energy of .511 MeV. Since the positron and electron are both briefly at rest during this annihilation, the system has no momentum during that moment. This is the reason that two gamma rays are created. Conservation of momentum would not be achieved if only one photon was created in this particular reaction. Momentum and energy are both conserved with 1.022 MeV of gamma rays (accounting for the rest energy of the particles) moving in opposite directions (accounting for the total zero momentum of the system). However, if one or both particles carry a larger amount of kinetic energy, various other particle pairs can be produced. The annihilation (or decay) of an electron-positron pair into a single photon, cannot occur in free space because momentum would not be conserved in this process. The reverse reaction

Pair production

Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...

 is also impossible for this reason, except in the presence of another particle that can carry away the excess momentum. However, in quantum field theory

Quantum field theory

Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

 this process is allowed as an intermediate quantum state. Some authors justify this by saying that the photon exists for a time which is short enough that the violation of conservation of momentum can be accommodated by the uncertainty principle

Uncertainty principle

In quantum mechanics, the Heisenberg uncertainty principle states a fundamental limit on the accuracy with which certain pairs of physical properties of a particle, such as position and momentum, can be simultaneously known...

. Others choose to assign the intermediate photon a non-zero mass. (The mathematics of the theory are unaffected by which view is taken.) This opens the way for virtual pair production or annihilation in which a one-particle quantum state may fluctuate into a two-particle state and back again (coherent superposition). These processes are important in the vacuum state

Vacuum state

In quantum field theory, the vacuum state is the quantum state with the lowest possible energy. Generally, it contains no physical particles...

 and renormalization

Renormalization

In quantum field theory, the statistical mechanics of fields, and the theory of self-similar geometric structures, renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities....

 of a quantum field theory. It also allows neutral particle mixing through processes such as the one pictured here.

Proton-antiproton annihilation

This form of annihilation occurs when a quark from a proton, and an anti-quark from an anti-proton collide with each other. The two particles collide and annihilate, which results in the creation of virtual gluons. A top and antitop quark emerge from the gluon cloud, and these quarks begin moving apart and stretching the gluon field between them. As the particles move away from each other, the top quark and antiquark decay into a bottom and antibottom quark, and emit W force carrier particles(a W- boson and a W+ boson). An electron and neutrino emerge from the virtual W- boson and and up quark and down antiquark emerge from the virtual W+ boson. At the end of this process the bottom quark and bottom antiquark, electron, neutrino, up quark, and down antiquark all move away from each other.

See also

• Pair production
  Pair production
  Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
  
  
• Electron-positron annihilation
  Electron-positron annihilation
  Electron–positron annihilation occurs when an electron and a positron collide. The result of the collision is the annihilation of the electron and positron, and the creation of gamma ray photons or, at higher energies, other particles:...
  
  
• Proton-antiproton annihilation
• Neutron-antineutron annihilation