Figure references are to the second edition of Modern Physics by Serway, Moses and Moyer (Saunders, 1997)

As a result of experiments which measure the results of the collision of cosmic rays or accelerated stable particles (such as the electron and proton) with matter, more than 400 "elementary" particles have been discovered. Many of these are unstable, with half-lives ranging from 10^-6 to 10^-23 seconds. The challenge to particle physicists has been to make sense out of this apparently complicated structure of the subnuclear world. Around 1970, this question was simplified with the discovery that many of the particles (not including electrons or photons) could be considered to be made up of subunits named quarks. To understand the properties of elementary particles (by implication, the basis of the entire natural world) we need to describe the forces (or interactions) between them. Four fundamental types of particle interaction have been identified.

The strong force is held responsible for binding quarks together to form protons, neutrons and other (relatively) heavy particles. This is a short-range force which becomes insignificant at particle separations greater than 10^-15 m. The nuclear force between nucleons is thought to be a residual effect of the much stronger force holding quarks together within each nucleon.

The electromagnetic force binds electrons and protons within an atom, and its residual effect holds together the atoms in a molecule or in a solid. This is a comparatively long-range force, decreasing proportional to the square of the distance between charged particles.

The weak force is a very short-range force (acting over distances < 10^-18 m) which is involved in beta-decay and decay of some elementary particles such as the neutron (halflife 920s). It is now believed that weak and electromagnetic forces are manifestations of a single electroweak force.

The gravitational force is a long-range inverse-square force which is negligible in the realm of atomic, nuclear and particle physics but which becomes the dominant force of attraction between large electrically-neutral objects separated by large distances, such as planets, stars and galaxies.

According to classical physics, the gravitational or electromagnetic forces are transmitted by the presence of a field produced by one object, to which a second object responds. This field can transmit energy and momentum between the two objects. In quantum field theory, the field is regarded as being quantised and the energy or momentum exchange is carried (or mediated) by field particles (to be distinguished from the matter particles which they link). In the case of the electromagnetic field, the field particle is a photon - usually referred to as a virtual photon since it is not normally observable directly. This photon exists for only a short time D t (typically <10^-16 s), having being created by one of the interacting particles, using a "borrowed" amount of energy D E in accordance with the uncertainty relation D E D t ~ h , before being absorbed by the second interacting particle. Likewise, a particle called the graviton (not yet detected experimentally) is held responsible for transmitting the gravitational field. The strong force is mediated by field particles named gluons, which exist for less than 10^-20 seconds.

Matter particles can be grouped according to the forces between them (i.e. the kind of field particle which can link them). All particles undergo gravitational interaction (although this is negligible) and those which are electrically charged can undergo electromagnetic interaction. Particles which are also capable of interacting via the strong force are named hadrons, and are divided into baryons and mesons. Baryons are relatively heavy particles, such as the proton and neutron, and are believed to consist of quarks held together by gluons. Mesons have medium values of rest mass and include the pion, which was once thought to be the field particle which transmits the nuclear force between the particles in an atomic nucleus.

Leptons are particles which can undergo weak interactions. They are the smallest known particles and comprise the electron, the muon, the tau particle and three kinds of neutrinos, together with antiparticles of each of the above.

In the late 1950's, so-called strange particles were discovered in cosmic-ray experiments. These particles were hadrons which were created through strong interactions but decayed relatively slowly (halflives in the range 10^-15 to 10^-10 s) via weak interactions. To explain their properties, a new quantum number S (strangeness) was introduced. S is conserved in particle decays which involve strong and electromagnetic interactions, but not those which involve the weak interaction.

Although leptons appear to be true elementary particles, posessing no size or internal structure, hadrons have both size and structure, as revealed by the results of collision experiments. It is possible to group baryons or leptons, according to their charge and strangeness, so that recognizable symmetry patterns are produced, such as the hexagonal grouping of eight baryons shown in Fig. 15.9a. Named the eightfold way by its originator (Murray Gell-Mann), this grouping also suggests the presence of an underlying substructure. In 1963, Gell-Mann proposed that each hadron consists of either two or three pointlike particles (quarks) whose individual electrical charge is either -(1/3)e or +(2/3)e. Thus, a proton consists of two positively-charged up-quarks and one negatively charged down-quark, giving a net charge of +e, whereas a neutron consists of two down-quarks and one up-quark, giving a net charge of zero; see Fig.15.11.

In addition to their up, down or strange properties, quarks can be distinguished by a colour charge which is analagous to electrical charge but is associated with the strong (rather than electromagnetic) force. Quarks are therefore labelled red, blue and green; the colours must be different for each quark in a hadron, making the particle itself white or "colourless". The underlying theory is called quantum chromodynamics (QCD) and according to this theory the strong force between quarks is mediated by massless particles called gluons which also carry a colour charge. Quarks (and gluons) have never been detected directly, as a particle tracks in photographic emulsion for example. However, their production is accompanied by directed jets of hadron particles which are detectable and act as a kind of signature; see Fig. 15.12. This non-observability property is known as confinement and is thought to arise from the fact that the strong force between quarks increases as the particles separate.

According to QCD, the quarks within a nucleon (proton or neutron) are constantly emitting and absorbing virtual gluons. If two nucleons approach within 1 fm of each other, gluons or quarks can be exchanged, which is the true origin of the nuclear force.

In 1979, the scientists Glashow Salam and Weinberg won a Nobel prize for introducing electroweak theory, according to which the weak and electromagnetic interactions become equivalent at very high particle energies. The combination of electroweak theory and QCD (which describes the strong interaction) is known as the Standard Model and represents the generally-accepted description of particle behaviour, which (so far) is in agreement with all experimental results; see Fig. 15.14.

One question not answered by Standard Model is why a photon has zero rest mass, whereas the field particles which mediate the weak force have a large mass (85 and 97 times that of a proton). Because of this mass difference, the weak and electromagnetic interactions are distinct under normal conditions and only become equivalent when the energies of the interacting particles exceeds that of the rest mass of these field particles. One suggestion is that a Higgs field (named after its proposer, the Scottish physicist Peter Higgs) may exist everywhere in space, and that particles derive their mass through interaction with this field. Associated with this field would be a Higgs particle, whose rest energy might be as large as 1 TeV (10^12 eV). To detect this particle, a particle accelerator giving very high energy (perhaps 40 TeV) would be required and does not currently exist.

Attempts have been made to unite electroweak and QCD theory, to produce a Grand Unified Theory (GUT) in which all forces (except gravity) are described in terms of a single mechanism. Some GUT theories predict that the proton is not completely stable but has a halflife of 10^31 years (about 10^21 times the age of the universe!). Experiments to detect proton decay are underway but have not yet yielded positive results.

One attempt at a Theory of Everything (TOE) which includes gravity (a goal which eluded Einstein) involves the concept of superstrings, exceedingly small entities with a diameter of the order of 10^-32 m. In superstring theory, equations are written in a large number of dimensions (typically 10), most of which are not directly observable because they are "rolled up" around the string. The theory is still under development, particularly with regard to the possibility of experimentally verifying any of its predictions.

For a popular but intelligent discussion of the history and future of string theory, consult: Superstrings: A Theory of Everything, ed. P.C.W. Davies and Julian Brown (Cambridge University Press, 1988).