The Standard Model

The Universe (seen through the eyes of particle physics)

Matter

Matter is everything that exists in the Universe – it is all the stuff that was created in the Big Bang. Particle physicists believe that matter is built of twelve types of fundamental particle – the building blocks of the universe. These fundamental particles cannot be broken down any further.

Diagram of the Standard Model

There are two families of fundamental particles – the quarks and the leptons. 

There are six sorts of quarks and six sorts of leptons. Together they make up a theory called the Standard Model.

Most matter on earth is made from a combination of two quarks, called the up and the down quarks and a lepton called the electron.

The up and down quarks form protons and neutrons inside the nucleus of the atom, and the electrons orbit the nucleus to complete the whole atom.

The rest of the twelve fundamental particles are more commonly found in high energy environments, for example in particle accelerator collisions, or right at the start of the universe just after the Big Bang.

Forces

We believe that there are four fundamental forces in the universe; gravity, and the electromagnetic, weak and strong forces.

We think the effect of gravity on fundamental particles is really tiny.  So we don't really consider it for the moment in particle physics.

The electromagnetic force affects any electrically charged fundamental particle (that's half of the leptons and all the quarks). It's the same force that makes lightning strike and different poles of bar magnets attract each other.

The weak force is responsible for radioactive decay. It actually makes neutrons turn into protons, amongst other things, and every type of matter particle experiences it.

The strong force (so-called because it is stronger than the weak force) is only felt by quarks.  It behaves like elastic, because the further apart you pull two quarks, the stronger the strong force gets between them.

Each force has one or more force-carrying particles associated with it. We think forces are felt by matter particles when force-carrying particles interact with them.

So what is left to find out?

Our theory of particle physics, The Standard Model, is a mathematical description of the 12 fundamental particles and 3 forces. We haven't yet found any experiment that disagrees with it, however hard we try.

However, there are lot of things that aren’t explained yet in particle physics.
For example:

• why are there exactly twelve fundamental matter particles?
• are these twelve particles fundamental, or are they in turn are made up of other, smaller particles?
• what is mass - how do particles get heavy?
• where does gravity fit in to the Standard Model?

So our understanding is clearly incomplete – in fact, we don’t know what 96% of the universe is made of - and that's why we do research!