Particle physics

Contents

What is particle physics?

Particle physics is the study of the fundamental constituents of matter and the forces of nature.

Science doesn’t get much bigger or more exciting than this. Particle physics research involves the biggest, most complicated experiments in the history of science, with the fastest computers, the coldest temperatures and the strongest magnets on Earth.

Particle physics re-creates the universe just after the Big Bang and hopes to answer the questions humans have been asking for eternity:

  • where do we come from?
  • what are we made of?

Atoms and particles

Particle physics is a journey into the heart of matter. Everything in the universe, from stars and planets, to you and the chair that you’re sitting on, is made from the same basic building blocks, particles of matter. Some particles were last seen only billionths of a second after the Big Bang. Others form most of the matter around us today.

Particle physics studies these very small building block particles and works out how they interact to make the universe look and behave the way it does.

How small is small?

Really small. Think about the width of a human hair, one of the smallest things we can see. Twenty of them placed side by side fit across one millimetre.

If we use a microscope to look inside a hair we see cells, which are formed from molecules. Each molecule is made up of a collection of atoms. We know that everything is formed from various types of atoms and that atoms are really small. You can fit a hundred thousand of them across a human hair.

But particle physics doesn’t stop there. We can see right inside the atom. We see that atoms consist of a nucleus, ten thousand times smaller than the atom, surrounded by a cloud of electrons. The nucleus is a collection of particles called protons and neutrons. And inside protons and neutrons we find particles called quarks.

Quarks are so small that we haven’t yet been able to measure how big they are. We just know that they are at least ten thousand times smaller than the nucleus. They are so small that we treat them like mathematical pinpoints in our theories.

Zooming down in scale from a person to a fundamental particle like a quark or an electron is like shrinking the diameter of the whole earth to the size of a 5p coin. And then shrinking the 5p by the same amount again. This is what we mean by really small.

How do we do particle physics?

We recreate the conditions just after the Big Bang, when particles roamed freely through the universe.

We do this with powerful particle accelerators, which accelerate particles close to the speed of light and smash them together. Particle physicists then look at what happens in the high energy collisions.

Particle physics is a bit like trying to find out how a watch works by bashing together two very expensive Swiss watches and then learning to rebuild them from all the bits of glass, cogs and springs.

In place of Swiss watches we use particles so small that you could fit about ten thousand million of them across a watch face and, despite their tiny size, the collisions between these particles have as much energy as a large aeroplane taking off!

The universe

Matter

Matter is everything that exists in the universe, 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.

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
  • electromagnetic force
  • the weak force
  • the strong force.

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 three forces. We haven’t yet found any experiment that disagrees with it, however hard we try.

However, there are a 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 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.

Particle accelerators

What is a particle accelerator and why do we use them?

Just after the Big Bang, the universe was a rapidly expanding ball of fundamental particles.

As the universe expanded, it cooled and the particles decayed, changing into other fundamental particles. These particles then joined together and gradually formed the matter that we see around us today.

In particle accelerators we smash beams of particles together in head-on collisions that are energetic enough to turn the clock back to just after the Big Bang. The more energetic the collisions, the more likely we are to make fundamental particles appear again.

Once we’ve produced fundamental particles, we can study their behaviour to find out why the universe is made the way it is.

What does a particle accelerator look like?

The biggest particle accelerator in the world is at CERN, the centre for particle physics research, just outside Geneva in Switzerland.

Above ground, you wouldn’t know anything about it, but if you were to go 100 metres underground, you’d find yourself in a circular tunnel, about the size of a London underground tube tunnel. This is where the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, is being built.

The LHC tunnel runs for about the same distance as the London Underground Circle Line, 27 kilometres in a ring underneath the French-Swiss border. If you were inside the accelerator tunnel you would see a tube which runs continuously in either direction. This tube is the ‘beam pipe’, so-called because inside here, two beams of particles fly round the tunnel.

The beams are accelerated to very high energies by magnets surrounding the beam pipe. These make sure that the two particle beams circulate in opposite directions without crashing into each other. When the beams of particles reach their final top energy, the magnets alter their path and bring them into collision at predetermined points around the accelerator ring.

By this time, the particle beams are travelling so close to the speed of light that they collide forty million times a second. Inside each collision we have a snapshot of the fundamental particles that last existed billionths of a second after the Big Bang. Now all we have to do is build gigantic particle detectors at each collision point to try and work out exactly what went on.

Particle detectors

If we’ve created particles in a collision in an accelerator, we want to be able to look at them. And that’s where particle detectors come in. We build these at the collision points in an accelerator and use them to identify as much of what was produced in the collision as we can.

The principle of a particle detector is simple. It will never ‘see’ a particle directly, but it shows where it has travelled, what signature tracks it leaves behind and the effect it has on the detector when it is stopped as it flies out of the collision.

Detectors consist of layers of different types of material, which are used to either show us the path of a particle as it travels along, or absorb it to make the particle stop.

We can identify different types of particles depending on where they stop in the detector and what their path of travel looks like. It’s a bit like a police investigation after a car crash. If we know what particles were produced in the collision, in which direction they flew and how much energy they had, we can reconstruct what exactly happened in the collision.

In particle physics, reconstructing the particle collision means you can find new types of particles and work out how they interact with each other.

What does a detector look like?

Big! Current experiments are as big as a house. ATLAS, an experiment that will run at the LHC, will be as big as a cathedral. Detectors have to be this big to stop highly energetic particles that were travelling near the speed of light.

From the outside, detectors look like huge boxes, with literally kilometres of cables attached. These cables carry electronic signals from inside the detector to computers outside to be processed. If you could see inside, you’d see onion-like layers of materials like silicon, plastic, steel, lead glass and lots of support structure to hold the whole thing together.

The data pouring out of these detectors will be analysed to answer fundamental questions about the way the universe works.

Last updated: 31 March 2022

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