# What is the antiparticle of a neutron

## Antiparticle

Elementary particles exist in two forms, as normal Particles and as Antiparticle. The two particles have opposite electrical charges and opposite parity. Their mass, their spin and their magnetic moment are identical. As normal the particle that occurs in the matter surrounding us is considered, and the antiparticle is the particle with the opposite charge. Antiparticles are the building blocks of antimatter. For example, the positron is the antiparticle of the normal electron. The electron is assigned the lepton number 1, the positron −1.

If a particle meets its antiparticle, there is a high probability of annihilation: Proton and antiproton annihilate into several pions, electron and positron annihilate into two or three photons. Conversely, a photon can be converted into an electron and a positron, which is known as pairing.

### theory

The concept of antiparticles arises from quantum physics, more precisely from quantum field theory. For reasons of symmetry, there is an antiparticle for every elementary particle, which is opposite to the particle in its additive quantum numbers such as charge (electrical charge, color charge, weak charge), baryon number, lepton number, etc. In contrast, the non-additive quantum numbers such as B. the spin, the mass, the lifetime, etc. are identical.

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

Antiparticles are identified as symbols with a dash, for example:

\$ \ p \$ - proton \$ \ bar {p} \$ - antiproton

### 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 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 positive energy \$ E = + m c ^ 2 \$ as well as with negative energy \$ E = - m c ^ 2 \$. The first question that arises is why a particle with positive energy does not change into the state of negative energy with radiation of \$ 2m c ^ 2 \$. Dirac's interpretation was that all negative energy states are occupied (Dirac sea). The pair formation is then the lifting of a particle from the negative to the positive energy state. The unoccupied negative energy state, the hole, becomes observable 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.

### literature

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