In 1932, with the discovery of the neutron, it seemed that the picture of the atomic world was complete. This picture featured four elementary particles (photons, electrons, protons and neutrons), the properties of which are summarized in Table 3.1. The atomic nucleus was composed of neutrons and protons, and the behaviour of the electrons surrounding the nucleus was well explained by quantum mechanics (which successfully explained many other phenomena in the atomic and sub-atomic world). Actually, this picture was not a perfect one. There were several unsolved problems such as the process of beta decay and the nature of the force holding the nuclear components together. The theories which attempted to solve these problems predicted the existence of additional particles, and the experimentalists searched for and discovered them — along with other particles that the theoreticians had not predicted. The particle research of the 1930s and 1940s is conveniently described under four headings:

1. (1) discovery of the positron and understanding the role of antiparticles;

2. (2) the neutrino and the ‘weak force’;

3. (3) Yukawa's theory of the ‘strong force’;

4. (4) discovery of the muon and pion.

The experimental research on the elementary particles in that period depended quite heavily on cosmic rays, which are briefly described in the next section.

Along with the exciting experiments that changed the face of physics in the twentieth century, work proceeded feverishly on the theoretical side to rebuild the conceptual world that had been severely shaken. Sometimes the theorists lagged behind the experimentalists, and certain experimental results waited for several years to be explained. And sometimes experiment lagged behind theory, and a new theoretical model waited for years to be verified experimentally. Two great theories left their mark on physics in this century: the theory of relativity, and quantum mechanics. In this chapter we shall briefly review the main ideas of these theories which are vital for the understanding of the atomic world.

The theory of relativity

The theory of relativity was formulated by the great physicist Albert Einstein. The first part of it (and the important one for us) — the special theory of relativity — was published in 1905, and sprang from the need to explain a puzzling question that exercised the physicists of the nineteenth century: why was it impossible to alter the speed of light, which remained constant (299 792.5 kilometres per second) even when the detector was moving relative to the source of light? In contrast to other physicists, Einstein did not try to explain this phenomenon.

The beginnings of atomic research

The word ‘atom’ is derived from the Greek ‘atomos’, meaning indivisible. In about the year 400 BC the Greek philosopher Democritus postulated that all matter was made up of minute particles which could not be destroyed or broken up. He was unable to perform any experiment to support his hypothesis, but this concept could account for the fact that different substances had different densities: the more the atoms were compressed the denser and heavier the substance became. A few Greek sages accepted the atomic theory of Democritus, but the great majority adopted the view of Aristotle, who believed that matter was continuous in structure, and this was also the opinion held by the alchemists in the Middle Ages. When modern scientific research began in the seventeenth and eighteenth centuries, the concept of atoms was revived and appeared in the writings of scientists, but it was generally mentioned incidentally and no attempt was made to use it to explain natural phenomena or to verify it experimentally.

John Dalton is regarded as the father of modern atomic theory. He was an English teacher who dabbled in chemistry as a hobby and became one of the founders of modern chemistry.

The first accelerators

In the 1930s and 1940s, cosmic rays were the only source of new particles. The high-energy cosmic particles are excellent projectiles for splitting nucleons and producing new elementary particles, and physicists indeed made good use of them for this purpose. However, the disadvantage of this source of radiation is that it cannot be controlled. All that can be done is to send instruments or photographic films into the upper atmosphere and wait for results. If one wishes to investigate a certain process, occurring at a particular energy, one has to sort through a very large number of photographs in order to find a few containing the required information, if at all. As the research of the particle world strode forwards, physicists wanted to imitate in the laboratory the processes produced by the primary cosmic particles. For this purpose they had to accelerate charged particles — protons and electrons — to high speeds with the aid of electric and magnetic fields. This transition ‘from hunters to farmers’ took place in the 1940s, although its beginnings were in the early 1930s.

The first machine for accelerating particles was built over the years 1928 to 1932 in Rutherford's laboratory (the Cavendish Laboratory) at Cambridge.

Conservation laws and symmetries

What are the laws which rule our particle jungle? Is it possible to comprehend why one particle or another decays in a given manner, or to predict the results of a collision between two particles? The developers of quantum mechanics showed us the basic path which we must take: wave functions, wave equations, and so on. But the theory is incomplete, as we have no a-priori knowledge of the interparticle forces. Moreover, even when these forces become known, we often find that the calculations, in practice, are too complicated.

A number of simple laws, known as ‘conservation laws’, provide us with great assistance in both these concerns. Each law of this kind states that a certain physical quantity cannot change during the course of reactions between particles. The quantity remaining after the reaction must be identical to the quantity present beforehand. We can illustrate this with examples from the field of economics. In a country in which money is printed only at the rate at which it is taken out of circulation the general amount of money in circulation remains fixed. An industrial firm whose incomes exactly equal its expenditures maintains constant internal capital.

The physicists who dealt with classical physics (the physical theories prior to the appearance of relativity and quantum mechanics) were aware of a number of conservation laws, derived from decades of experimentation.

We can divide the brief history of research on elementary particles into a number of periods, among them times of bewilderment and confusion and others of astounding discoveries; periods characterized by feverish study concentrated on a single subject, and periods during which progress was made on many fronts. During the years between 1947, the year the first V-tracks were discovered, and 1953, when the ‘strangeness’ idea was first proposed, experimental efforts were concentrated on the study of strange particles. Towards the end of the 1950s and during the early 1960s interest turned towards a new type of extremely short-lived particles, which were termed resonances.

Extremely short-lived particles

The reader who has gained some insight into the various conservation laws, and has understood how certain particles — such as strange particles — manage to live so long, thanks to conservation laws which prohibit decay by strong interactions, may now ask: Can't energetic collision (between protons for instance) also produce particles which are not barred by any conservation law from decaying by the strong interaction? If such particles do exist, their life expectancy is of the order of only 10-23 seconds (the time required for the effect of the strong force to traverse a distance equal to the diameter of the particle).

In the 1960s and 1970s the list of hadronic resonances rapidly expanded. Physicists watched its growth with mixed feelings. On the one hand the many new particles would provide them with a rich field of investigation for many years to come, but on the other, the inability to explain the connections between the various particles pointed only too clearly to the lack of a comprehensive theory concerning the nature of sub-atomic particles. The classification into four families no longer provided a means of bringing order into the chaos, since two of the families — the mesons and baryons — had flourished, and eventually added tens upon tens of new members to their number. In the late 1950s and early 1960s several attempts were made to draw up a new classification scheme which would divide the two large hadronic families into sub-families on the basis of the quantum numbers of the particles. Such a scheme was needed not only as an aid to finding one's way around the jungle of particles, but it could also be the first step towards finding a satisfactory theory which would explain the existence and properties of all the particles.

Experience had shown that when certain observations could not be fitted into a comprehensive theory, it was often useful to sum up the abstracted regularities in heuristic formulae or illustrative tables, even if their theoretical basis was not yet clear.

Biologists who study the animal or vegetable kingdoms made use of a classification system which breaks down the hundreds of thousands of species into various groups, classes and families, according to generally accepted criteria. Were it not for the elucidation of this classification system, by Carl von Linné (Linnaeus) in the eighteenth century, Darwin would not have been able to conceive of the theory of evolution and biologists would have encountered great difficulties in doing research and in communicating their findings to their colleagues. While the number of various particles we have encountered so far is by no means as overwhelming as the number of living species, the overall picture at this stage may seem a little confusing to the reader who is making his first acquaintance with the world of elementary particles. Undoubtedly, the physicists who began discovering particle after particle felt the same way. A system of classification was thus essential in particle physics, as well. When we sort the particles into various categories, not only does the general picture become clearer, but the physical laws which reign over the jungle of particles also become increasingly apparent. In previous chapters we occasionally hinted that one group of particles or another constitute a single family.

Much has happened in particle physics since the last (1989) printing of The Particle Hunters. Some of the new discoveries have filled critical gaps in the picture of the world which particle physicists have endeavoured to sketch in the last four decades. This picture, although by no means perfect, is much more complete now than it was a few years ago. Therefore, when Dr Simon Capelin from Cambridge University Press wrote to us, asking whether we would be willing to produce a new and updated second edition of the book, it was difficult to decline.

In undertaking that arduous task we were encouraged by warm responses of readers from all over the world, who have read the book either in English or in some other language (Spanish, Italian, Japanese or Hebrew) and wrote to tell us how they had benefited by it.

A gentleman from Sussex sent us his own theory of elementary particles, which he had conceived after reading our book. A young journalist from Eire who managed to embrace all the laws of physics in one equation, published it in a local newspaper and sent us the relevant issue. A poet sent us a poem in which she recounted the dreams she had after finding out what the world is made of, and a public relations manager in an accelerator centre in the USA told us how the book had helped her to understand the operation of the accelerator and to explain it to visitors.

We here discuss dynamical systems in which the ground state does not possess the same symmetry properties as the Lagrangian. When this happens in certain field theories one finds that there inevitably exist massless scalar bosons, the so-called Goldstone bosons. Remarkably, however, when this happens in a local gauge theory involving massless vector fields and scalar fields, the would-be Goldstone bosons disappear, but contrive to re-emerge disguised as the longitudinal mode of the vector fields, which thereupon behave like massive vector bosons with three spin degrees of freedom. In this way the unwanted massless vector bosons of the gauge theory are replaced by heavy vector mesons as demanded by the phenomenology of the weak interactions.

It is well known that a considerable simplification obtains in a problem whenever the interaction possesses some symmetry. Exact symmetries, such as electric charge conservation, are, however, fairly rare in nature, and the usual way of representing the situation is to assume that a small piece of the Lagrangian violates a particular symmetry whereas the rest of ℒ is invariant. Thus strong interactions conserve parity, isospin and strangeness, whereas electromagnetic interactions violate isospin, and weak interactions violate isospin, strangeness and parity, so that a hierarchy of forces results. A very interesting situation occurs when the solutions of a problem are not symmetric in spite of the Lagrangian being exactly symmetric—in particular if this is so for the ground state of the system. One then talks, somewhat inappropriately, of a ‘spontaneously broken symmetry’. The most celebrated classical example is a ferromagnet.