Why study particle physics? - A brief introduction

This section is only intended as a brief introduction to a number of the key aspects of particle physics, it is therefore a somewhat simplified and incomplete description. For those interested there seems to be extensive material on the subject of particle physics on the web; some useful links are given throughout this site. For relevant texts see the 'references' section'.

Particle Physics is an important branch of Physics in that it enables a picture to be built up of what matter is and how it works. It allows us to address questions such as 'What is the world made of?' and 'What holds the world together?' Click here to find out more

The world is made up of fundamental building blocks. Fundamental is used in the sense that these so-called building blocks are simple and structureless and cannot be made up of anything smaller.


Atoms consist of a nucleus surrounded by a cloud of electrons. Atoms can be arranged in the Periodic Table. Atomic nuclei have a charge +Ze, where e is the magnitude of the charge on an electron and Z is the atomic number. Z determines the number of atomic electrons and hence the ordered position of that species of atom in the periodic table. Elements are arranged (within the periodic table) left to right and top to bottom in order of increasing atomic number. This order generally coincides with increasing atomic mass. The periodicity of the periodic table is explained beautifully by the conservation rules of quantum mechanics. As it is possible to build a periodic table this suggests that atoms must be made of simpler building blocks. Atoms, therefore are not the most fundamental particles.

Protons and Neutrons

The nucleus is made up of protons and neutrons. The proton is a positively charged particle having a mass of 1.673x10-27kg whereas the neutron has no charge and a slightly(!) heavier mass of 1.675x10-27kg. The proton and neutron both have a radius of 10-15m. The proton and neutron are held together by the strong nuclear force.

Nuclear physics is another branch of physics in which the physics of the atomic nucleus is studied. Nuclear physicists are concerned with understanding the properties of nuclei in terms of the behaviour of their constituents. Click here to find out more


Protons and neutrons are not fundamental as they consist of sets of point-like particles called quarks held together by the exchange of messenger particles called gluons. Current understanding suggests that quarks and gluons have no sub-structure. Scientists believe that quarks, electrons and a few more particles (see the standard model, below) are fundamental. Elementary Particle Physics is concerned with studying these fundamental particles.

Figure 1 shows a schematic representation of atoms, electrons, protons, neutrons and quarks.

Figure 1: Schematic representation of quarks, protons, neutrons and electrons

Particle physicists look for new particles and on finding them categorise them in order to try and find patterns of how fundamental building blocks of the universe interact.

The Standard Model

The Standard Model is the name given to the model which particle physicists have at present to describe the 200 or so elementary particles and their interactions. This model is very successful and can explain all the particles using 6 quarks, 6 leptons and force carrying particles. There are two elements to the model:

(1) The Building Blocks

These are the six quarks and six leptons (see types of particle). These can be arranged into three families (known as generations!):

1st Family (Generation) 2nd Family (Generation) 3rd Family (Generation)
up quark (u) charm quark (c) top quark (t)
down quark (d) strange quark (s) bottom quark (b)
electron muon tau
electron neutrino muon neutrino tau neutrino

(2) The Interactions

There are 4 interactions, each having associated with it one or more force carrying particles, these are what hold the 'building blocks' together:

Gravitational Weak Electromagnetic Strong
Exchanged particles graviton???* W+, W-, Z0 photon gluon
Mass heavy zero zero

* The gravitational interaction has no important effects in particle physics and so is only mentioned above for completeness. It is thought an exchange particle called the 'graviton' may exist but this has not been found yet!

Of these three interactions of interest (weak, electromagnetic and strong) the weak and electromagnetic are manifestations of the same interaction, the electroweak interaction. The electroweak interaction requires the masses of the weak and electromagnetic exchange particles to be massless, this is clearly not the case. It is thought the W and Z particles acquire their mass by interacting with a new type of field, the Higgs field. The Higgs field predicts at least a pair of Higgs particles, these are the particles which will be sought for in the Large Hadron Collider which is due to open at CERN in 2005.

Although a good theory, the standard model is incomplete as there is experimental evidence which the Standard model cannot explain.

Experiments in particle physics are carried out at giant particle accelerators and their associated detection equipment. The accelerators are required to accelerate particles to high energies for two reasons:

(1) In order to investigate particles of the size 10-18m, radiation is required with a wavelength (l) comparable to that of the particle being studied. From de Broglie's relation it can be seen that a tiny radiation wavelength demands a very high energy (E):

E = h c / l

where h is a constant, known as Planck's constant.

(2) Many of the fundamental constituents have large masses and require correspondingly high energies for their creation and study. This can be seen from Einstein's famous equation

E = mc2

Where E is the energy, m is the mass of the particle and c is the speed of light. It can readily be seen that a large energy is required to create a particle of large mass.

For a more in depth discussion of many of the points discussed above, click here

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