# Antiparticle

Each type of elementary particle exists, as far as is known, in two forms, as 'normal' and as **antiparticles** , which can, however, be identical. As far as is known, there is complete symmetry: the antiparticle of the antiparticle is again the original particle. For example, the positron is the antiparticle of the normal electron and vice versa. The mass , life span and spin of a particle and its antiparticle are the same, as are the nature and strength of their interactions. On the other hand, electric charge , magnetic moment and all charge-like quantum numbers are oppositely equal. The electron has the lepton number 1, the positron −1. The parity of particles and antiparticles is the same for bosons and opposite for fermions . Particles whose charge-like quantum numbers are all zero are their own antiparticles.

Because of the perfect symmetry, antiparticles can unite to form antimatter just like normal particles to form normal matter .

If a particle and an antiparticle of the same type of particle come together, there is a high probability that annihilation occurs : Proton and antiproton annihilate into several pions , while electron and positron annihilate into two or three photons . Conversely, a photon can be converted into an electron and a positron, which is called pair formation .

## theory

The concept of antiparticles arises from quantum physics , more precisely from quantum field theory . It is based on the CPT theorem , according to which the field equations do not change due to a CPT transformation for very fundamental reasons. This is the combination of a sign reversal of all types of charges ( charge conjugation , C), a reflection of space ( parity , P) and a reversal of the direction of time ( time reversal , T). Because of this *invariance* (immutability) there is a second similar state or process for every state or process that is possible according to the field equations, which emerges from the first through the CPT transformation and is just as possible. If the initial state contains only one particle, the CPT transformation results in the antiparticle in the corresponding state. If the state describes a whole system of several particles, the corresponding state of a system results which is built up like the original one, but from the corresponding antiparticles.

Because of this CPT symmetry , a corresponding type of antiparticle can be expected for every type of elementary particle, which is opposite to the particles in its additive quantum numbers such as charge (electrical charge, color charge), baryon number , lepton number , etc., in its non-additive properties such as e.g. B. spin , mass , life , etc. but identical. These antiparticles have been proven experimentally for all known types of particles.

If all additive quantum numbers of a particle are zero, the particle is its own antiparticle. This is e.g. B. the case with the photon , with Z ^{0} and with the neutral pion π ^{0} .

## Notation

The normal name is "Anti-" followed by the name of the particle, for example antiproton; a historically created exception is the name positron for the antielectron. For some particles, such as muon or pion , the sign of the charge is usually used instead of “anti-”, ie “positive muon” or “my-plus” instead of antimuon.

In formulas, antiparticles are usually marked with a slash: for proton , for antiproton . However, the positron is almost always written, the positive muon and pion mostly or , and correspondingly for other short-lived particles.

## history

The first known antiparticle was the positron, which was theoretically predicted by Paul Dirac in 1928 and discovered by Anderson in 1932. The antiparticles of the other two constituents of stable matter, the antiproton and the antineutron , were discovered in 1955 and 1956, respectively.

## Interpretations

The Dirac equation , which describes electrons, among other things, has solutions with both positive energy and negative energy . The first question that arises is why a particle with positive energy does not change into the state of negative energy when it is emitted. Dirac's interpretation was that all negative energy states are occupied ( Dirac sea ). Pairing is then the lifting of a particle from the negative to the positive energy state. The unoccupied negative energy state, the hole, can be observed as an antiparticle.

The interpretation with the help of the Dirac lake was replaced by the Feynman-Stückelberg interpretation . This is based on the idea that particles with negative energy move backwards in time. Mathematically, this is equivalent to an antiparticle with positive energy moving forward in time.

## See also

## literature

- Lisa Randall:
*Hidden Universes*. Fischer Verlag, Frankfurt am Main 2006, ISBN 3-10-062805-5 .

## Individual evidence

- ^ PAM Dirac: The Quantum Theory of the Electron . In: Proceedings of the Royal Society of London. Series A . No. 778 , 1928, pp. 610-624 , doi : 10.1098 / rspa.1928.0023 ( online ).
- ↑ CD Anderson: The Positive Electron . In: Physical Review . tape 43 , no. 6 , 1933, pp. 491–494 , doi : 10.1103 / PhysRev.43.491 ( online ).

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