# Force Carrier Particles Fact File

Updated on February 15, 2013

## Physics for Beginners - What are Force Carrier Particles?

This hub aims to summarise the facts you should already know about force carrier particles and their interactions.

In order for you to apply the facts that follow in this hub, you will need to have already learned about the fundamental particles that comprise our universe.

If you haven't already done so or need to recap, see:

## Four Interactions

There are four interactions that occur between particles.

1. Electromagnetic
2. Strong
3. Weak
4. Gravity

Every force that we know of can be explained with these four fundamental interactions.

## Force Carrier Particles

• The exchange of four types of force carrier particles are responsible for the four interactions.
• Only matter particles that can be affected by a force carrier particle can emit or exchange that type of force carrier particle.
• For example, a neutron does not have an overall electromagnetic charge and therefore can neither absorb nor emit photons - the force carrier particle for electromagnetic force.

## 1. Electromagnetism (EM)

• EM force causes positively and negatively charged particles to be attracted to oppositely charged particles and repelled from same charged particles.
• Friction and magnetism are caused by EM force.
• Photons are the force carrier particles that facilitate EM force.
• Photons have zero mass and travel at the speed of light (300 million meters per second) in a vacuum.
• Atoms usually have the same amount of electrons as neutrons and so are electrically neutral, but, since different sides of the atom have different charges (due to where electrons are at the time) a force called the residual electromagnetic force is able to bind different atoms together into molecules.

## 2. Strong Force

Strong Force and Colour Charges

• Strong force holds together the quarks inside baryons (e.g. protons and neutrons) and mesons.
• Strong force works through the relationship between colour charged particles.
• The force carrier particles that carry strong force are called gluons.
• Gluons have colour charge and so do the particles that they affect: quarks and anti-quarks

Quarks and Colour Charges

• Composite particles made out of quarks have no net colour charge since all of the quark colour charges in the composite particle cancel each other out.
• When quarks are in close proximities, they exchange gluons with each other and this creates a colour force field that binds the quarks together.
• There are three colours a quark can have and three anti-colours an antiquark can have. If a mixture of all three quarks are present in a composite particle, then the particle is said to be colour neutral. This is what baryons have.
• A particle can also be colour neutral if it contains a quark colour and its corresponding antiquark colour e.g. blue and antiblue charges. This is what mesons have.
• Emitting or absorbing a gluon always results in the colour change of the quark that emitted/absorbed and thus colour charge is conserved.
• Therefore, a gluon is said to have both a colour and anti-colour charge.
• Although there are 9 combinations you could potentially have with one type of the three quark colours and one type of the three antiquark colours, in reality there are only eight combinations. No reason has yet been found for this.

Quark Existence

• Quarks and antiquarks cannot be found individually, instead they form colour neutral threes (like in Baryons) or twos (like in mesons).
• No other number of quarks can be found because the resulting composite particle would not be colour neutral.
• The exchange of gluons is what keeps quarks together and this feature is given the visualisation of 'colour foce field'. If a quark is taken away from its neighbouring quark (with whom it is exchanging gluons, creating a 'gluon force field') then the force field stretches until it becomes more energy efficient to use the energy from the forcefield to make a new complementary quark which can then exchange its own gluons with the wandering quark. Therefore, it is impossible for a quark to escape being next to a complementary quark(s).
• What keeps protons and neutrons together in a nucleus is the residual strong force between the quarks found in those protons and neutrons. This force is stronger than the electromagnetic force pushing the protons and neutrons away from each other.

## 3. Weak Force

• The Standard Model uses the term electroweak interaction to describe both the electromagnetic and weak interactions.
• At about 10-18 meters the strength of weak interaction is about the strength of electromagnetic interaction.
• At 3x10-17 (30 x the distance of 10-18) the strength of the weak interaction is 1/10,000th of electromagnetic interaction.
• The weak and electromagnetic forces have equal strength. However, due to the mass of their force carrier particles (photons are massless whilst weak interaction carriers are massive) and the distances at which they work, their observed strengths are different.

• Although there are six kinds of quarks and leptons, stable matter is only made out of the least massive quarks and leptons (up and down quarks and electrons), as well as the neutrinos.
• These small particles cannot decay any further and this is why stable matter is made out of them.
• More massive particles like muons and tao leptons decay into these stable particles as well as kinetic energy.
• When a quark or lepton changes into a different type of quark or lepton it is said to change flavour. These flavour changes are a result of the weak interaction.
• The force carrier particles for the weak interaction are known as the W+, W- and the Z particles.
• The Z particle is neutral and the W particles are charged.

## 4. Gravity

• The Standard Model cannot actually explain Gravity.
• By simple observation we can see that gravity exists in our universe with some degree of universality.
• The force carrier particle for gravity has been given the name 'graviton' but no gravitons have ever been observed.
• The Standard Model is still able to make accurate predictions about fundamental particles because gravity has only a very small effect in most particle interactions.

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