I want to avoid going in to a complete history of this subject, but it appears that I must provide, at least, some background information. The history of particle physics is a chaotic collection of patches, fixes and quilt-work. Occasionally, great discoveries were made and misinterpreted or misunderstood. Theories about the make-up of matter have been around since mankind could speak, but many of those have had to be corrected or replaced as new data emerged-especially nowadays, when theories are being knocked down almost as soon as they come along, not for failing to explain the current understandings, but because of new discoveries. New scientific paradigms also seem to crop up every few years and thinking in physics is forced to change again.

 Atoms

The concept of particles, as the basic constituents of matter, reaches back to the ancient Greek civilization, much of which's culture, mathematica and science has been lost through the centuries at the hands of the shortsighted and corrupt. Because of this, it is uncertain who of the Greeks was the first to suggest it, but the name atomos or ατoμoσ (in Greek), meaning "the smallest, indivisible thing" is ascribed to Democritus around 585 B.C. The idea here, of course, was that if one were to cut down a bit of matter, and then cut it again, and then again, continually cutting it down further, eventually, one would reach a point where the particulate could no longer be cut. This was the smallest indivisible thing, and it was natural to believe that such a thing had to exist¹.

Between 585 B.C. and the late 1700's there was actually very little advancement in the natural philosophy (science) of particles. Of course, much was lost due to wars, famine, pestilence and politics. The library at Alexandria was said to be the greatest collection of scientific knowledge in the world. Of course, in those days, the gathering of human knowledge and understanding was not recognized as a worthwhile pursuit, and the library was burned to the ground. (This may have actually happened several times-history books do not seem to agree on this.)

The science of modern chemistry, like modern physics, has grown out of an age-old art. Before there were chemists, there were alchemists whose great purpose in life was to turn lead into gold. In those days, the modern alchemists were somewhat more scientific than their predecessors, who were magicians and sorcerers. Alchemy, in its time, was seen by many as a study not too far removed from witchcraft.

John Dalton was probably the first great chemist, since he was the first to suggest that different elements had similar properties and that when some elements were combined in different ways with other elements, they produced materials with similar qualities. In 1808, he set the foundation for the modern view of atomic theory. This gave us the first clue that there may, indeed, be one or more very small bits of matter from which all other forms could spring.

In 1869, Dmitri Mendeleev suggested that many of the elements they knew of had very similar properties. Though some elements were much heavier than others, they could be arranged according to these properties in a table-later to be known as the Periodic Table of the Elements (which any student of modern chemistry should be thoroughly familiar with). From this, scientists could easily see that there were definitely only a certain number of different types of matter, and today, there are known to be about 104 elements, the last few of which are highly unstable and can only be produced in laboratories.

Later, in 1895, W. Roentgen, while working with a Cathode Ray Tube (CRT), discovered rays that could easily penetrate through many materials. He had made the discovery of X-rays, which, within months after, were being used in medicine. This did not actually support or provide any conclusions about the nature of matter, except that certain forms of light could be transmitted through it.

J.J. Thomson, in 1897, was the first person to use the CRT (a description of which will be provided in detail shortly) at very low vacuum pressures to discover that cathode rays could be deflected by an electric field. Thomson calculated the kinetic energy of this deflection and, supposing that cathode rays were actually only very small particles, measured their mass and discovered an extremely small, negatively charged particle which was to be named the electron. This, of course, was a major breakthrough in the science of particle physics and, if not for the theories generated by Einstein, whose name has easily overshadowed the names of many great scientists, Thomson would probably be very famous today. Instead, his name and his fantastic discovery remain relatively obscure to the general public.

Becquerel is credited as being the first person to discover radioactivity, in 1896-quite accidentally. He actually came across several forms of radiation while experimenting with the phenomenon of phosphorescence. This, of course, dealt mostly with material which, after being exposed to sunlight, would glow in the dark. Becquerel had inadvertently left some radioactive material near a protected photographic plate and found later that images had formed on the plate without ever having been exposed to light.

Between the years of 1896 and 1900, Ernest Rutherford did more work on this and identified three different types of radiation which he named α (alpha), β (beta) and γ (gamma) rays. α-Rays were actually found to be heavy, positively-charged particles, identified as helium nuclei (which is a helium atom-two protons and two neutrons- stripped of its electrons). β-rays were eventually found to be high-energy electrons and γ-rays were simply found to be very high-energy photons (or light rays).

Atoms, at this time, were believed to be a cloud-like homogeneous mixture of "stuff," (often thought of as having a "pudding-like" consistency) which, of course, included several of Thomson's electron particles. In the year 1909, however, all of this changed when one of Rutherford's students, Ernest Marsden, while firing electrons at a thin wafer of gold foil found that, while the vast majority of the electrons merely passed through the wafer-some being deflected at odd angles-a very small number of them actually bounced back. Rutherford, shocked by this anomaly, eventually reasoned that atoms were composed of a very large amount of empty space (comparatively speaking) with a very small, heavy core, later named the nucleus. Positively charged particles were now seen to occupy this nucleus while electrons orbited about it in a cloud. This discovery, and the resulting subsequent theories was, in its impact, comparable to Thomson's discovery of electrons.

So important was the discovery of the nucleus, that a short time later, Niels Bohr was able to put together the work of Rutherford, Louis de Broglie and Max Planck to come up with what is now known as the Bohr Model of the Atom, which, hopefully, we are all familiar with. This model, however, in its early stages, suggested that there were equal numbers of charged particles inside and orbiting the nucleus. The particles inside were positively charged and given the name proton and for every proton in the nucleus, there was a negatively charged electron in orbit about it.

This theory was okay for a while except that it did not jibe well with the current understandings of elemental make-up. Scientists had already discovered many of the elements which comprise our present-day tables and knew, fairly well, what their individual masses and charges ought to be. In Bohr's model of the atom, masses and charges did not add up properly, and so a third, uncharged particle, called a neutron, was conjectured, which could make up the difference in the masses without disturbing the total charge.

Neutrons were first discovered by Irene Joliet-Curie (daughter of Marie Curie) and Frederic Joliet, but misinterpreted by them as the X-rays, which had been discovered by Roentgen. James Chadwick repeated their experiment in 1932 and determined that the "X-rays" that the Joliets had uncovered were actually large, uncharged particles with (about) the same mass as protons.

Even so, there were still difficulties with the model concerning electric repulsion. Since, in electricity, opposites attract and likes repel, it did not make any sense that all of the protons could collect in the nucleus without expelling one-another. It had already been shown, by the use of Quantum Mechanics, that the Bohr model of electron orbits could remain stable (not lose energy and spiral down to the nucleus) indefinitely. (The use of Quantum Mechanics in this model is crucial, since without the quantization of energy, orbiting electrons would continually lose energy as a result of their motion and spiral down to the nucleus, thereby causing the atom to collapse.)

No one seemed to take notice of this oddity until 1935, when Japanese physicist Hideki Yukawa published a paper suggesting the existence of another particle which would mediate what he called the nuclear strong force, thus holding the nucleus together. Using electrodynamics, Yukawa calculated this particle to have a mass of about 140 MeV, and christened it the pi meson (π or pion). This was actually quite a large suggestion to make, since no one, up to this point had been considering the existence of any other types of particles besides protons, electrons and neutrons. Yukawa, also the bearer of a rather obscure name in physics, could easily be hailed the "father" of modern particle theory.

Of course, the π meson had never been seen, but two physicists, Carl D. Anderson and Seth H. Neddermeyer, using a cloud chamber as a detector, discovered something similar to it in 1937, and thought that they had actually found it. Not too long afterwards it was decided that this was not Yukawa's particle, but in fact, an entirely new particle never before seen. The new particle was called the mu meson (μ or muon).

The cloud chamber was the invention of C.T.R. Wilson in 1911. What it is, essentially, is a (usually glass enclosed) chamber containing a supersaturated vapor and it works like this;

A supersaturated vapor is one that is ready-actually past the point-to form liquid droplets, but requires some small impurity around which droplets can form. In nature, there are usually so many impurities in the air (dust particles, etc.) that droplets form quickly and easily-as soon as the vapor reaches saturation density. If there are no impurities (and also very little motion) for droplets to form around, a vapor can actually become supersaturated, in which case, even the slightest disturbance will cause the droplets to form.

The advent of the cloud chamber made it possible to detect sub-atomic particles, since, whenever a loose particle traversed the chamber it left a trail of droplets. Depending on what type of particle it was, it left different kinds of trails. If the particles entering the chamber were moving very fast, they would knock electrons loose from some of the floating molecules, causing them to become ionized, thereby creating a small disturbance in the vicinity of their trails.

Heavy particles left thick trails and light particles left thin ones. If you placed the cloud chamber in a large magnetic field, charged particles would be deflected according to their masses and velocities. From this, their masses and charges could be roughly determined.

Of course, this was not the only way to detect small particles of matter. Particle detection on the atomic, and even the sub-atomic scale, is an art-science within itself, and has grown over the years out of bits and pieces of discoveries, many of which, when first found, were not understood. I do not want to go too far into the subject of particle detection in this book, but there are some things that need to be stated about it.

When physicians first started using Roentgen's X-ray discovery to see things inside human patients, they were never in doubt of what they saw. If the X-ray photograph showed pictures of bones in a human skeletal structure, they knew that what the photo showed was true, because they had dissected humans (which is required of medical students) and knew that there was a skeleton beneath the flesh. Also, they could press their fingers into the flesh and feel the bones-they knew they were there. The X-rays merely confirmed what they already knew.

This is not necessarily the case with physics. In particle physics (in the metaphorical sense), we are allowed only to see the X-rays and then must surmise that there may be bones beneath the flesh. We are not allowed to actually feel the bones or even see them from dissecting a corpse-we must guess. This is the main problem in dealing with the extremely small; we are invariably limited by our means of detection. No one has ever actually seen an electron or a proton, and certainly no one has ever actually witnessed a photon in flight.

And so we are limited as to what we are able to surmise about the things we do see. It is very much like asking the question, "What lies at the core of the earth?". The absolute answer to this is that we do not know, quite simply, because no one has been there to look and see. But we are able to make excellent guesses based on things we do know and theories about the way things should be. Of course, if our theories are incorrect, then when we do find out new things, they do not always turn out the way we think they should have been.

The science of particle physics is very much this way, and it is highly dependent upon our means of detecting particles. About our means there is little we can do, unless we can discover new ways of detecting very small particles. In this, the only way we can err is in our theoretical basis'. This is usually a vision beyond the perceptual-beyond what we can physically see and experience. In this sense, when we look at matter through the "eyes" of our detectors, we are looking through the "microscope" of a logical, coherent structure; it is a difficult microscope to look through. In other words, it is easy to make mistakes.

Another type of particle detector was invented by Donald Glaser, called a Bubble Chamber. Glaser was watching bubbles form on the inside of a glass beer mug and knew that the little stream of bubbles were being formed around irregularities on the inside surface of the mug. From this, he got the idea that if you were to heat a fluid such as liquid hydrogen or such, to near boiling point, then at the exact right instant, lower the pressure on the fluid, this would lower the boiling point slightly and bubbles would form wherever impurities existed. If the impurity happened to be a fast-moving sub-atomic particle, then it would leave a track of bubbles.

This type of detector is superior to the cloud chamber, because the denser the material, the more things there are for very small particles to "bump" into. Some very small particles can go all the way through a cloud chamber and never bump into anything; this is also the case with bubble chambers, but it is less likely. Also, similar attributes can be measured, such as mass and charge, in the same manner as in cloud chambers.

These, of course, are not all the types of detectors there are in the world, but I do not really want to go into the other types, since all I actually wanted to do was introduce the ideas behind the problems of detection in general, and show how this could lead to other problems.

Perhaps this was not the next major discovery in particle physics, but Yukawa's π meson was finally discovered by C.F. Powell in 1947. It had a mass of about 139 MeV, and appeared to match up perfectly with the properties described by Yukawa. Also, in the same year, several other types of particles were discovered. But these seemed, in a sense, to be closely related to pions-particularly in their decay modes. They were named kaons (K) and had masses of about 500 MeV. There were four kinds eventually discovered: K⁺, K- , K⁰ and ⁰. But these types of particles also had another strange property, and so, had the property known as strangeness; they were known as strange particles.

Of course, the only thing peculiar about the property of strangeness, was that these particles were extraordinarily long-lived-on the (rough) order of 10- ¹⁰ seconds, while other particles were much shorter lived-on the (again, rough) order of 10- ²³ seconds.

The property of strangeness is extremely peculiar in that, some particles exhibit this property directly and are consequently given the strangeness value of +1-which is done rather arbitrarily. All kaons (K-particles) are given the strangeness value of ±1 and all pions (π-particles) are assigned the strangeness value of 0 (zero). In order for strangeness to be conserved in a reaction, however, other particles were also assigned values of strangeness and this, also, is done rather arbitrarily. In other words, the value of strangeness applied to most particles is based upon extrapolation rather than direct experimental evidence.

Many properties about matter were being observed through their respective decay modes. A decay mode is, quite simply, the manner in which a particular type of particle decays into another type or other types of particles. For example, a free neutron cannot last more than a few minutes outside of a nucleus and eventually decays into three other particles: a proton, an electron and a neutrino. The expression for this is

n ® p⁺ + e- + v_e (5.1)

In this equation, several properties of the various particles involved are said to be conserved. Charge, for example, is conserved in this decay, since, on the left side of the equation, the total charge is zero (0). On the right side, if the charge of an electron has a unit value of -1, then the proton has a charge of +1 and the neutrino has a charge of zero, thus giving the right side of the equation a net charge of zero, which matches the left side. This decay mode was known about before any actual discoveries of other particles were made. (As a note to this, I might add that, in all known reactions to date, charge is always conserved.)

Another thing conserved is momentum (which is the reason for the [postulated] neutrino-this is discussed later). When a neutron decays, the proton in the reaction shoots off in one direction, the electron in another and the neutrino in yet another. When the three momentums are added vectorially, the total momentum adds to zero (assuming, of course, that the neutron had no momentum to start).

Strangeness is also conserved, but only certainly in strong decay processes. It is also conserved in only some weak interactions. For example, in the process above, protons, neutrons, electrons and neutrinos are all said to have strangeness equal to zero, and so, in this decay, strangeness is conserved.

Of course, using bubble and cloud chambers (and other types of accelerating and detecting devices), and the ideas of conservation, many new particles seemed to be cropping up all over the place in different types of experimental facilities (which will also be discussed shortly), and eventually, a host of new particles was discovered. This new "conglomeration" was huge and confusing, and every time a new particle was discovered, several theories about the nature of matter collapsed. Two men, however, almost concurrently devised a new method of classifying this conglomerate, although only one of them is very well recognized for it.

Murray Gell-Mann is usually thought of when people talk about the eightfold way, but another physicist who worked with Gell-Mann, Yuval Ne'eman, was also involved in this development, which took place around 1960-61. Ne'eman's name has also become obscure, even to many physicists.

The eightfold way, of course, refers to Buddhism, an ancient chinese religion originally formulated by The Buddha, which professes the "naturalness" of opposites, as the cohesiveness necessary for life and the existence of the universe. Great physicists are typically a symbolic sort, and often have a flair for the dramatic (or even the romantic-cases in point: charm, truth and beauty² quarks) and this is the reason that we sometimes end up with weird phrases like the eightfold way.

Their method was to organize individual particles-particularly hadrons (I will explain these shortly)-into hexagonal patterns, the particles representing the points on and in the hexagon, according to charge and strangeness. Table 5.1 shows the hadrons that Gell-Mann and Ne'eman had to work with in 1960-61.

Table 5.1 A table of the known hadrons at the time when Gell-Mann and Ne'eman developed the eightfold way patterns of matter using the conserved properties of charge and strangeness. Hadrons are believed to be the mediators of the nuclear strong force.

When these particles are arranged according to their respective charges and strangenesses, a pattern emerges which can be helpful in understanding the make-up of matter (which it did-which, in turn, is how the idea of quarks came about). Figure 5.1 shows this pattern for mesons.

Figure 5.1 Hexagonal patterns show a pictorial representation of the eightfold way for mesons.

Of course, what can be done with mesons can be also done with baryons. Baryons are, quite simply, massive particles such as protons and neutrons. By this time however, a fairly long list of baryons had evolved from the discoveries made at the big accelerator laboratories. (In the back of this book [in the appendices] can be found a complete, [almost] up-to-date listing of all the known mesons and another of all the known baryons.) Figure 5.2 is the eightfold way pattern for baryons developed by Gell-Mann and Ne'eman.

Figure 5.2 Eightfold way for baryons. Strangeness is measured along the vertical axis and charge is measured diagonally.

At the time of the original development of this pattern, the omega minus (Ω- ) particle (which is shown in parenthesis to denote that it was not originally a part of the pattern) had not yet been discovered. Its existence was later theorized independently by both Gell-Mann and George Zweig. A short time after their separate predictions, the particle was discovered (it needed only to be looked for).

Eventually, both Gell-Mann and Zweig realized that all of the hadrons known of at the time could be seen as being constructed of three basic particles which Gell-Mann called quarks; these were named up, down and strange. The up and down quarks represented the direction of isospin (I) for the quark-i.e. the up quark was named for isospin up and the down quark was named for isospin down. The strange quark was said to be part of the make-up of particles which exhibited the property of strangeness.

 Go to Chapter 6

Home Begin Preface Acknowledgements Contents Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Appendix A Appendix B1 Appendix B2 Appendix C1 Appendix C2 Appendix D Appendix E Appendix F Appendix G General References Future Books About the Front Cover About the Author Index