Interaction was described in terms of the exchange of photons between particles and the nuclear force in terms of the exchange of mesons between nucleons. These processes involve the creation and annihilation of the exchanged photon or meson by the interacting particles and, as already mentioned, the strength of the interaction, which is related to the probability of creation and annihilation processes, is measured by the square of the relevant coupling constant. It is clear from the relative size of these quantities that the nuclear interaction is around 100 times stronger than the electromagnetic interaction. It is an example of what is generically called the strong interaction, which, is experienced by a great many particles. We also encountered the beta decay creation and annihilation processes involving nucleons as well as electrons, neutrinos and their corresponding antiparticles. The probability of such processes happening, as indicated by their mean lifetimes, indicate that the coupling constant responsible for the creation of, say, electrons and antineutrinos in beta decay is very small indeed many orders of magnitude less than the electromagnetic coupling constant. For this reason, beta decay and many other similar processes are said to derive from the weak interaction. In the light of these comments it is now possible to classify the many different elementary particles met within the physical world in terms of their masses, spins and the interactions they experience. Very broad classifications are as follows.

Photon. This is the massless quantized manifestation of an electromagnetic field and only experiences the electromagnetic interaction. It has spin 1 and, is an example of what is referred to as a gauge boson.

Leptons. The collective name for the very small number of particles such as electrons and neutrinos which only experience the weak and, if electrically charged, the electromagnetic interaction. They all have spin ½.

Hadrons. The collective name for particles which experience the strong interaction as well as the weak and, usually, electromagnetic interactions. They are divided into two groups.

Mesons. These are those hadrons, like the pion, which have integer spin (usually 0 or 1).

Baryons. These are those hadrons, like nucleons, which have half integer spin (usually 1/2 or 3/2).

Apart from those elementary particles which have been encountered so far, it is now known that there are literally hundreds more which fall into one or other of the above categories. These have largely been discovered using high energy accelerators. The basic mechanisms for accelerating charged elementary particles were mentioned. Over the last 50 years accelerators have been built producing beams of particles with higher and higher energies. These energies are measured in electron volts and, to put accelerator energies into perspective, remember that the energy needed to knock a typical nucleon from a nucleus is around 8MeV. Accelerators used in nuclear physics usually have energies around 20-40MeV, sufficient to knock several nucleons out of a nucleus, but in investigating elementary particles much higher energies are used. For example, the Super Proton Synchrotron at CERN (European Centre of Nuclear Research in Geneva) is a circular machine having a circumference around 6 km and producing protons of energy 450,000MeV (or 450GeV). In such an accelerator the protons are accelerated by radio frequency electromagnetic fields and kept in essentially fixed orbits by powerful magnetic fields whose value increases as bunches of protons continually accelerate. The Large Electron Positron Collider (LEP) at CERN accelerates electrons and positrons in opposite directions in circular orbits with a circumference of 27 km. They then collide with each other, the energy of the collision being up to 190 GeV. The tunnel of LEP is also to accommodate what is known as the Large Hadron Collider (LHC) in which protons will be accelerated in opposite directions and collide with each other at energies which will eventually reach around 14,000 GeV. Linear machines are also in use, for example the Stanford Linear Accelerator (SLAC) in the USA, in which electrons are accelerated over a distance of around 3 km to energies of 50 GeV. When charged particles are accelerated in, for example, circular machines they continually emit electromagnetic radiation. This is known as synchrotron radiation and some machines are specifically designed to produce this radiation as an intense source of x-rays for use in experimental studies in many fields of physics. High energy particle accelerators are massive undertakings and can generally only be financed on an international basis. Their immense energies do mean that it is possible to create particles of masses much larger than the heaviest particles we have encountered, namely the neutron and proton, whose mass energy (E = mc²) is around 1 GeV. Hundreds of new particles have been created in this way and are found to be very unstable. They decay very quickly and it is for this reason that they have not appeared directly in the physical processes we have dealt with so far. They have been identified using detectors of the type already together with other devices such as, for example, bubble chambers and, more recently, wire chambers. The former are large chambers containing low temperature liquified gases such as hydrogen near to boiling which are such that, when a charged particle passes through, local boiling takes place and bubbles along its track are formed. The way in which such tracks bend in a magnetic field and also their lengths then enable the mass and mean life of the particle to be deduced. Wire chambers contain positively and negatively charged wires immersed in an appropriate gas. Charged particles entering the chamber knock electrons out of the gas atoms and the resultant ions then go on to knock further electrons out of other atoms. The electrons in the resulting pulse are attracted towards a positively charged wire, giving a large signal. Analysis of such signals then enables the position and momentum of the incoming particles to be measured very accurately. To give some impression of the various particles which have now been identified, just a few of them in each of the categories referred to earlier together with their names, symbols (frequently Greek letters), approximate masses (as multiples of the electron mass m_e), approximate mean lives and the most probable products of the decay. Also, for hadrons, one other quantity known as strangeness is given; this will be explained in the next section. It should also be remembered that corresponding to each particle is an antiparticle having the same mass and spin as the particle and with an opposite and equal value for quantities such as electric charge. Antiparticles are denoted by adding an overbar to the particle symbol.