# The Standard Model

Our best theory of all the particles in the universe and how they interact

## What is the Standard Model?

The Standard Model is a mathematical model of all the fundamental particles in our universe and how they interact. It was first devised in the early 1970s when it brought together everything that we knew about particles at the time. It also predicted the existence of additional particles too.

There are 17 particles in the Standard Model and these can be divided into two groups: fermions and bosons.

All the matter in the universe is made up of a series of fundamental particles called fermions.

Fermions are divided into quarks and leptons. They make up protons, neutrons, atoms, molecules, you and me.

Fermions obey a rule called the exclusion principle. This simply states that fermions cannot occupy the same place at the same time. Which is something you’re probably aware of if you’ve ever walked into a wall.

There are 12 fermions in the Standard Model. Six quarks and six leptons.

The six quarks are divided into three pairs of two. Scientists call these pairs “generations”.

The lightest and most stable particles (up and down quarks) make up the first generation. The heavier and less stable particles belong to the second and third generations (charm and strange, top and bottom quarks).

The six leptons are also arranged in three generations: the electron and electron neutrino (first generation), the muon and muon neutrino (second), the tau and tau neutrino (third).

Fermions interact with each other through another set of fundamental particles called bosons.

Bosons can quite happily occupy the same place at the same time. They are also the carriers of the four fundamental forces of the universe.

They include gluons (carrying the strong nuclear force), photons (electromagnetic force), and W and Z bosons (weak nuclear force).

The graviton is a hypothesised carrier particle for gravity. But it’s never been observed experimentally and is not included in the Standard Model.

Last but not least is the Higgs boson, which produces a field that interacts with particles and gives them mass.

## So what?

The Standard Model is widely regarded as one of the most successful theories in physics. Its mathematical description (called the ‘Lagrangian’) describes the interactions between elementary particles.

It’s undeniably brilliant and when it was first devised in the 1970s, it didn’t just tell us about the particles we’d already detected. It also predicted particles we didn’t know existed at the time, helping scientists look in the right places and understand the universe a little bit better.

## What else?

The Standard Model is incomplete. It doesn’t include dark matter, it doesn’t explain dark energy and only incorporates three out of the four fundamental forces.

There’s also one pretty big problem with the Standard Model. It also predicts matter and antimatter should have been produced in equal amounts at the start of the universe.

If that was true, matter and antimatter would have annihilated each other and the universe would just exist as a sea of light. (Don’t panic, there was slightly more matter than antimatter. That’s why you’re sitting here reading this.)

The Standard Model also predicts neutrinos should have zero mass. And they don’t.

So, physicists are hunting to find a complete theory. New information from experiments in particle accelerators (such as the Large Hadron Collider at CERN) could help us fill in some of the gaps.

If these experiments detect new particles that don’t fit in the Standard Model, we could come up with a complete model.

## Want more?

Particle physicist Jon Butterworth takes us through the quantum world and the Standard Model:

#### Let’s have a coffee with the Standard Model!

A great paper from the IOP (Institute of Physics) diving into the mathematics of the Standard Model.

#### The Standard Model from the Physics Hypertextbook

One of the best write-ups on the Standard Model and fundamental forces by Glenn Elert.

#### The New Cosmic Onion by Frank Close

A briliant book, unlocking the secrets of the universe’s particles – and the universe itself.

### Special relativity

Quantum Mechanics
November 8, 2019

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