# Antiparticle

Many types of elementary particles exist in two forms, as normal particles and antiparticles as. For example, the positron is the antiparticle of the electron normal. Mass, lifetime and spin of a particle and its antiparticle are equal, also the type and strength of their interactions. In contrast, electric charge, magnetic moment, and all charge -like quantum numbers are equal and opposite. Thus, the electron has the lepton number 1, the positron -1. The parity of particles and antiparticles are the same for bosons, fermions at opposite. Particles whose charge -like quantum numbers are all zero, are their own antiparticles.

Due to the perfect symmetry of antiparticles can be just as much antimatter close together as the normal particles of normal matter.

Meet a particle and an antiparticle of the same particle together, it comes with a high probability of Annihilation: proton and antiproton annihilate several pions, electron and positron annihilate to two or three photons. Conversely, a photon into an electron and a positron are converted, one speaks of pair formation.

## Theory

The concept of the antiparticle arises from quantum physics, more precisely from quantum field theory. It is based on the CPT theorem, which states that the field equations of very fundamental reasons must be invariant under a CPT transformation. Which is the combination of a sign inversion of all types of charges ( charge conjugation C), a reflection of the room ( parity P) and a reverse time direction ( time reversal, T). Due to this invariance is provided for each condition or process that is possible according to the field equations, a second state or the like process, which is evident by the CPT- transformation of the first and is equally possible. Contains the initial state only one particle, is given by dieCPT transform the antiparticle in the corresponding state. Describes the state of a whole system of a plurality of particles, results in the corresponding state of a system, which is constructed as the original one, but from the corresponding anti-particles.

Because of this CPT symmetry is to be expected at any type of elementary as a corresponding type antiparticles, which is opposite in their additive quantum numbers such as charge (electric charge, color charge ), baryon number, lepton number, etc. the particles in their non-additive characteristics such as spin, mass, lifetime, etc. but identical. These antiparticles have been proven to all known species of particles experimentally.

Are all additive quantum numbers of a particle is zero, the particle is its own antiparticle. This is, for example, the case with the photon, the Z0 and the neutral pion π0.

## Notation

The normal term is " anti - " followed by the name of the particle, ie for example antiproton; a historically established exception is the name for the anti- electron positron. Some particles, such as muon or pion, instead of " anti - " usually called the charge sign, ie " positive muon " or " My- plus " instead antimuon.

In formulas antiparticles are usually marked with a slash: for proton, for antiproton. The positron is almost always written, the positive muon and pion usually respectively, and according to other short-lived particles.

## History

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

## Interpretations

The Dirac equation, which describes, among other electrons, both solutions with positive energy and negative energy. First This raises the question of why a particle does not move with positive energy with emission of negative energy in the state. Dirac's interpretation was that all negative energy states are occupied ( Dirac sea ). The pairing is then the lifting of a particle from the negative into the positive energy state. The unoccupied negative energy state, the hole is observable as antiparticles.

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