Fundamental Particles

In a nutshell

The four fundamental forces of the universe are gravitational, electromagnetic, weak and strong nuclear and they govern how particles interact and decay. Hadrons are particles and antiparticles that are affected by the strong nuclear force whereas leptons are not. Antiparticles are particles with the same mass but opposite charge as their hadron and lepton counterparts.

The four fundamental forces

The four fundamental forces govern how particles interact and decay; these are the gravitational force, the electromagnetic force, the weak nuclear force and the strong nuclear force. Each of them has a different strength and range and affects different particles.

force	What does it do?	relative strength	range
Strong nuclear	Holds all nucleons together.	$1$	$\sim10^{-15}\ m$
Electromagnetic	Acts on moving and static charged particles.	$10^{-3}$	$\infty$
Weak nuclear	Is responsible for beta decay.	$10^{-6}$	$\sim10^{-18}\ m$
Gravitational	Acts on all particles with mass.	$10^{-40}$	$\infty$

Note: Beta decay is a type of radioactive decay where a beta particle is emitted by an atom.

Hadrons and leptons

There are two categories of subatomic particles:.

Hadrons- these are particles that are affected by the strong nuclear force. They are further divided into baryons and mesons and include protons and neutrons. They decay through the weak nuclear force.
Leptons- they are not affected by the strong nuclear force. They include electrons, neutrinos and muons.

If charged, the electromagnetic force will act on on either of these types and, although negligibly, they are also affected by the gravitational force since they have mass.

Antimatter

The term antimatter refers to matter composed of antiparticles. These are a counterpart to particles and have the same mass but an opposite charge and are usually denoted with a line above. An example is the positron, the positively charged counterpart of the electron. One can even find antimatter atoms as shown below:

Physics; Particle physics; KS5 Year 12; Fundamental particles

When a particle and its antiparticle meet they destroy each other in a process called annihilation and produce a pair of photons, massless particles of light represented by the letter $\gamma$ . Antiparticles are affected by the same forces as their particle counterparts.

The image below shows an example of annihilation with an electron positron pair destroying each other to produce two photons:

Physics; Particle physics; KS5 Year 12; Fundamental particles

It is common to use relative masses and charges. These are mass and charge values that use the proton as standard:

Particle	Symbol	Relative mass	relative charge
Proton	$p$	$1$	$+1$
Antiproton	$\overline{p}$	$1$	$-1$
Neutron	$n$	$1$	$0$
Antineutron	$\overline{n}$	$1$	$0$
Electron	$e^{-}$	$0.0005$	$-1$
Positron	$e^{+}$	$0.0005$	$+1$
Photon	$\gamma$	$0$	$0$

Nuclear equations

Nuclear equations are used to depict how particles interact. The key to them is that when an interaction occurs some quantities will always be conserved, an example being charge. This means that the sum of all the charges needs to be the same on both sides.

Example

A proton and an antiproton annihilate each other and produce two photons in an interaction:

$p+\overline{p}\rightarrow\gamma+\gamma$ 

Show that charge is conserved in this interaction.

Firstly write down the charges of all the particles in this interaction:

$\begin{aligned}p&=+1 \\\ \overline{p}&=-1\\\gamma&= 0\end{aligned}$ 

Write the charges below the equation:

$\begin{aligned}p+\overline{p}&\rightarrow\gamma+\gamma \\1+(-1)&\rightarrow0+0\end{aligned}$ 

Finally compare the sums on both sides:

$\begin{aligned}1+(-1)&=0\\0+0&=0\end{aligned}$

Charge is conserved in this interaction.