# A-level Physics (Advancing Physics)/The Standard Model

The standard model of particle physics attempts to explain everything in the universe in terms of fundamental particles. A fundamental particle is one which cannot be broken down into anything else. These fundamental particles are the building blocks of matter, and the things which hold matter together.

The standard model is usually represented by the following diagram:

The particles in the standard model can be put into two groups: fermions and bosons. Fermions are the building blocks of matter. They all obey the Pauli exclusion principle. Bosons are force-carriers. They carry the electromagnetic, strong, and weak forces between fermions.

## Bosons

There are four bosons in the right-hand column of the standard model. The photon carries the electromagnetic force - photons are responsible for electromagnetic radiation, electric fields and magnetic fields. The gluon carries the strong nuclear force - they 'glue' quarks together to make up larger non-fundamental particles. The W+, W- and Z0 bosons carry the weak nuclear force. When one quark changes into another quark, it gives off one of these bosons, which in turn decays into fermions.

## Fermions

Fermions, in turn, can be put into two categories: quarks and leptons. Quarks make up, amongst other things, the protons and neutrons in the nucleus. Leptons include electrons and neutrinos. The difference between quarks and leptons is that quarks interact with the strong nuclear force, whereas leptons do not.

## Generations

Fermions are also divided into three generations. The first generation contains the fermions which we are made of - electrons, the up and down quarks, and the neutrino. The first generation particles have less mass than the second, and the second generation particles have less mass than their respective third generation particles. The second generation (the μ generation) contains two leptons: the muon and the muon-neutrino. It also contains the charm and strange quarks. The third generation (the τ generation) contains another two leptons: the tau and the tau-neutrino. Its quarks are the top and bottom quarks.

## Antiparticles

Every fermion has its antiparticle. An antiparticle has the same mass as a particle, but the opposite charge. So, the standard model contains 12 quarks, 12 leptons, and the bosons (which are even more complex).

## Questions

1. The third generation top quark was the last quark in the Standard Model to have its existence proven experimentally (in 1995). It is also the most massive of the quarks. Why was it so difficult to observe a top quark?

2. What observable phenomena does the Standard Model not explain?

3. How much more massive is an up quark than an electron?

4. How many fermions are there in the Standard Model?

5. The antiparticle of the electron (e-) is the positron. What is the charge and rest mass of a positron?

6. what difference do you find between bosons and quarks?