Objectives

• To understand the term fundamental particle
• To understand the standard model and how different particles differ or are related.

Fundamental Particles
A fundamental particle is one which can not be broken up into anything smaller, they are not made up of constituent parts. The atom was once thought of as a fundamental particle, the idea being that you could keep breaking a lump of matter up into smaller and smaller pieces until it would not split anymore. The name of this smallest ‘bit’ was termed the atom and would have been named a fundamental particle.
Since the atom was shown to made up of smaller pieces known as protons, neutrons and electrons, these three particles would then have taken the titles of fundamental particles (until this was otherwise proven incorrect) and the atom would no longer have been referred to as such.

The Standard Model
The Standard model has been used by physicists since 1970 and was originally proposed as a model to include all fundamental particles. Over the last century the concept of fundamental particles has developed significantly. Originally it was thought that an atom was a fundamental particle, but as we have seen from studying the history of the atom (insert link to the history of the atom), the atom is made up of smaller particles again – protons, neutrons and electrons. It turns out that protons and neutrons are made up of smaller particles again and as such protons and neutrons are not fundamental particles.

Two scientists, Murray Gell-Mann and George Zweig, developed the concept of the quark to help explain some issues with the pre-existing model of the nucleus. A few years later, it was shown with the use of the Stanford Linear Accelerator Centre (SLAC) that these smaller particles did in fact exist – giving rise to evidence for the existence of quark – since then a total of 6 quarks have been identified.

The standard model is a system used to help show a symmetry in matter. Similar to the periodic table, a system used to portray the different elements and to put them into ‘groups’ of similarity, the standard model attempts to do the same but using fundamental particles.

Below is an image of the standard model, which you need to familiarise yourself with;

In this model there are several groups, and new terms/ names, that are important to identify and learn for this model to become better understood. The two major groups in this table are fermions and bosons;

Fermions

These, as shown in the image of the standard model, make up the three generations of matter and include both the quarks and the leptons.

Bosons

Bosons include fundamental particles such as photons, gluons and W & Z bosons. They are their own class of particle as they are the force carrying particles and are responsible for the weak interaction (this will be covered at a later stage). The Higgs boson and hypothetical graviton are also types of bosons.

Fermions are broken down into two sub categories, quarks and leptons, both of which have three generations;

Quarks

Quarks are subatomic particles that contain a fraction of the elementary charge. They also make up anything that is known as a Hadron, these will be discussed later. Each generation of quark has its own ‘up-type’ or ‘down-type’. As can be seen from the table, the first generation of quark contains the up, u quark and and the down, d quark, the second generation contains the charm, c quark and the strange, s quark, the third generation contains the top, t quark and the bottom, b quark.

The charge of each ‘up-type’ of quark is $+ \frac{2}{3}$, whereas the ‘down-types’ each have a charge of $- \frac{1}{3}$. You should also notice that the spin of every quark is $\frac{1}{2}$ – for more information on a particles spin click here.

Leptons

A lepton is a type of fermion and carries an integer number of the elementary charge, either  $-1$  ,  $0$  or  $+1$  (as they are fermions, they do not include bosons). They also are subatomic particles but they do not take part in the strong interaction (the strong force does not apply to them). Each generation of lepton is linked somewhat with the generations of quark, they also come in two different classes, those that are charged and those that are neutral. How they link to the generations of quark will become more apparent when studying nuclear decay.

The charged leptons include the electron, $e$ the muon, $\mu$ and the tauon, $\tau$, the neutral leptons are known as neutrinos and are paired to the corresponding charged leptons. So, the electron is ‘paired’ to the electron-neutrino, $\nu_{e}$, the muon is ‘paired’ to the muon-neutrino, $\nu_{\mu}$ and the tauon is ‘paired’ to the tauon-neutrino, $\nu_{\tau}$ . You should also notice that the spin of every lepton is also $\frac{1}{2}$ (similar to quarks).

The above information gives you an introduction into the different types of fundamental particles that we are currently aware of. There is, of course, much much more information on each type of particle and category, the above however is the initial information you require to know for your A-levels.