# Introduction and Pair Production and Annihilation

## Introduction and Pair Production and Annihilation

#### Matter and antimatter

We live in a Universe composed of matter particles (e.g. the neutron, proton and electron etc.) However, antimatter particles are routinely created in particle accelerators.

All particles have antimatter counterparts. Anti-particles resemble their corresponding particles in every way except for the sign of their charge and the direction of their 'spin'. When an anti-particle meets its corresponding particle the two annihilate each other converting their mass to pure energy.

The three particles and their antimatter counterparts (plus their properties) you need to know are: the electron and positron; proton and antiproton; neutrino and antineutrino.

One of the mysteries of the Universe is why there is any matter at all when it is believed that equal quantities of matter and antimatter were created in the 'Big Bang'.

#### Pair production and annihilation

When a particle and its corresponding antiparticle meet they annihilate one another. These annihilations do not occur in a purely random way however; they must obey 3 rules: the conservation of energy, the conservation of momentum and the conservation of charge.

For example: consider the annihilation of an electron e- and its antiparticle the positron e+ (these are written as β- and β+ and in beta decays).

The combined mass is converted into pure energy in the form of photons. (2 gamma-ray photons are necessary to conserve momentum) and charge. Since the charges cancel at the beginning before the collision there is no net charge at the end either.

The reverse process is also possible. Of course the possibility of two photons of the right energy meeting to produce an electron/positron pair is negligible. However, a single gamma-ray photon can spontaneously produce such a pair as it passes close to a nucleus, which recoils thus conserving energy and momentum.

(in the presence of a nucleus)

The only requirement is that the gamma-ray photon carries an energy equivalent to the combined rest masses of the electron and positron. If the incident photon has more energy than this then the excess appears as kinetic energy of the electron/positron pair.