Particles, antiparticles and annihilation

In a nutshell

All particles have a corresponding antiparticle which possesses the same mass but an opposite charge. When a particle and its antiparticle meet they annihilate each other and produce two photons.

Antiparticles

All particles have an associated antiparticle and they share the same rest mass energy but have opposite charges. The rest mass energy is the energy one would obtain if all the mass turns into energy and is usually measured in $MeV$ .

Most antiparticles have the same name as their counterparts but with the prefix anti at the beginning and they are denoted by the same symbol but with a bar on top, though these are not always the case. Some of the most common particles and antiparticles are shown below.

Particle	Symbol	antiparticle	symbol
Proton	$p$	Antiproton	$\overline{p}$
Neutron	$n$	Antineutron	$\overline{n}$
Electron	$e^{-}$	Positron	$e^+$
Neutrino	$\nu$	Antineutrino	$\overline{\nu}$

In the following table are the charges and rest mass energies of the particles and antiparticles:

particle	relative charge	antiparticle	relative charge	rest mass energy
$p$	$+1$	$\overline{p}$	$-1$	$938.3\ MeV$
$n$	$0$	$\overline{n}$	$0$	$939.6\ MeV$
$e^{-}$	$-1$	$e^+$	$+1$	$0.511\ MeV$
$\nu$	$0$	$\overline{\nu}$	$0$	$>0.12\ eV$

Annihilation

When a particle and its antiparticle meet they destroy each other in a process called annihilation and produce two photons represented by $\gamma$ . Just like for any other interaction, the energy, momentum and charge is conserved.

An example of this process is shown below where an electron and a positron pair annihilate each other:

Physics; Particle physics; KS5 Year 12; Particles, antiparticles and annihilation

The process above can be represented as a nuclear equation in the following way:

$e^-+e^+\rightarrow\gamma+\gamma$

You can check that the charge is conserved in the interaction by summing the charge of the particles and show that it is the same on both sides:

$\begin{aligned}e^-+e^+&\rightarrow\gamma+\gamma \\Q:(-1)+(+1)&\rightarrow0+0\\Q:0& \rightarrow0\end{aligned}$