Quarks

Frontiers in Elementary Particle Physics

Yoichiro Nambu

• Publisher: WSPC
• Year: 1985
• Print ISBN: 9789971966669
• eBook ISBN: 9789814338028
• Topic: Physical Sciences
• Subtopic: Nuclear Physics

The book explains in a precise and complete manner how elementary particle physics has evolved over the past 50 years. The historical development of the ideas that have shaped our thinking about the ultimate constituents of matter is traced out. The author has been associated with some of the originators of elementary particle theory and has made significant contributions to the field. Here, he gives a first-person description of some of the main developments leading to our present view of the universe.
Contents:

• What is an Elementary Particle
• Quarks and Leptons
• Looking for Quarks
• Various Accelerators
• The Birth of the Yukawa Theory
• The Appearance of New Particles
• Orderliness of Elementary Particles and Conservation Laws
• Parity and Conservation
• Composite Models of Hadrons
• The Quark Model
• Evolution of the Quark Model
• Charm and its Followers
• Quarks with Strings Attached
• What is a Parton
• Tomonaga's Renormalization Theory
• QCD — The Quantum Mechanics of Color
• Spontaneous Breaking of Symmetry
• The Leaning Structure of the Weak Interaction
• The Weinberg-Salam Theory
• The Unified Field Theory
• Program for Grand Unification

Readership: General.

1

WHAT IS AN ELEMENTARY PARTICLE?

Questions that may not have Answers

Most people, even those particularly interested in physics, have heard the term “elementary particle” somewhere. For example, an editorial column of the Asahi newspaper is entitled “elementary particle”. Of course there is no relationship between the “elementary particle” newspaper column and the elementary particles we speak of in physics; yet such a title must have been chosen because it is a curiously attractive term. Why is it so attractive? This must be because the tradition of Japanese elementary particle theory, beginning with Hideki Yukawa’s meson theory, is perceived by the people as something of which to be proud.

The Yukawa theory was born when I was a high school student. The term elementary particle had already begun to be used widely when I had begun to major in physics at my university; and thus several of my fellow students and I went to our professor and told him of our desire to study elementary particle theory.

In chemistry the word “element” is used to signify the basic building blocks of chemical substances; “element” implies that everything is made from it but nothing makes it up. Perhaps this kind of theory is born naturally when Man begins to think about his environment; but the modern scientific concept of elements is influenced heavily by the natural philosophy of the ancient Greeks.

What, then, is an element exactly? This question can be expressed in two parts. First, is there indeed such a thing as an element? And second, if there is such a thing as an element, what is it? It must be obvious by now that the aim of elementary particle physics is to answer these questions.

But the answer is not so easy. One doesn’t even know if there really is an answer. Before the modern natural sciences there was a theory that earth, water, air, and fire were the elements. This theory was put forward by Aristotle; in English they still say that the influence of the natural world on Man, especially the damage done by rain, thunder, earthquake, and so forth, is caused by the “elements”.

Of course, there is no one today who thinks that earth, water, air, and fire are the true elements. The philosophy of natural sciences which demand verification would force us to actually take matter and divide it until we reached the smallest constituents. But even if we did this literally we would not get very far. If we cut matter with a knife and examined matter with a microscope, we would never reach anything like a basic constituent. Particles about 1 micron in size still have shapes and seem to have internal structure; but we can not get to the next smaller scale using knives or microscopes. After all, the knife itself is made of elements, and therefore there is no way to make a knife sharper than the elements themselves. Since we need the size of the knife edge to be smaller than the things we wish to examine, we are back to the same question.

It was not until the 19th century when the science of chemistry made rapid progress that the idea of an atom was born. The ancient Greek philosopher Democritus theorized that all matter was made up of unchanging elements called atoms (atom = a-tom = not-divide, i.e., not divisible), but the atoms we speak of in chemistry were naturally conceived from the study of chemical reactions. This process of discovery was nothing like the naive notion of mechanical division I spoke of earlier, but was based on fundamentally different principles. We turn next to these principles.

Something Unchanging

If we mix some chemical substances A, B, C, D, . . . chemical reactions will take place. For example:

and so on. We want to find out the laws that govern the results of mixing these substances; to that end, let us see if there are any patterns in these reactions.

Let us say that we found, as the result of our examination, that the masses of the substances remain the same before and after the reactions. That is to say, if we change the interpretation of the above equations and think of A, B, etc., as the masses of substances A, B, etc., those equations become ordinary mathematical equations.

Here we have discovered one conservation law of nature — the law of conservation of mass. Substances may react together and change their character completely, but there is some quantity remains unchanged; this is the meaning of a conservation law.

Now let us suppose that after a more detailed study of the above reactions we learned the following: In order for the reaction (a) to take place, A and B must be mixed in the 1 to 2 ratio by mass. For example, if we try to mix equal quantities of A and B by mass, half of A is always left over. In the same way, the substances in the reaction (b) also have fixed mass ratios among A, B, E, and F. In other words, the masses of the substances that participate in the reactions have integer ratio relationships.

Thinking about how the above situation arises is nothing other than tracing the development of chemistry in the 19th century. Everybody knows the answers to these problems now, but it is easy to imagine the following train of thought. If there is something (substance) that can be represented by the smallest integer in many chemical reactions, then aren’t all other substances made up of these smallest units? Doesn’t the fact that the ratios are integers indicate that the substances in these reactions are themselves made of such fundamental units? Then let us name this fundamental unit “atom” and try to apply our new theory to other situations . . .

Chemistry, in reality, is much more complex. There is not just one kind of atom; the hydrogen atom is the lightest, but there are many heavier atoms (elements) that seem to have masses in integer multiples of the hydrogen mass, yet cannot be broken up into hydrogen atoms. If we study more closely, the integer multiple relationships of the atomic masses are not exact; even the conservation of mass is not strictly adhered to. These and many other problems begin to arise.

Is It Real?

What about the question of the existence of these atoms? There is no doubt in the mind of the man who does chemical experiments that there are such things as elements that exist as matter. Hydrogen, nitrogen, carbon, etc., can be actually isolated. But the atoms are a different story; we can’t even tell the size of one atom. We couldn’t possibly look at it with our eyes — we don’t seem to be very far from Democritus’ idea of the atom.

But there are big differences. First of all the chemical idea of the atom is based on a quantitative law and can be verified by experiment. Secondly, as research progresses, the existence of the atom begins to be proven from many different directions. We can find the size of the atom, and in a certain sense, we can see the atom with our own eyes.

For example, the tracks left by particles in a bubble chamber give the “illusion” that we can actually see the atoms. A man who was taught that the size of the atom is about 10⁻⁸ cm may immediately conclude that he couldn’t possibly see an atom and the tracks must be an illusion; but is this correct?

Although we can see this book in front of us, we can’t simply say that this book exists because it is large. What enters our brains is the stimuli of the light that is reflected by the book and not the book itself. When a charged particle enters a bubble chamber it excites and ionizes the liquid of the chamber; these ions become seeds for the bubbles and when these bubbles become sufficiently large, they reflect light and become visible. Now the processes involved in seeing tracks in bubble chambers are somewhat more complex than those involved in seeing a book, but can we say that they are fundamentally different?

To answer the question “What is existence?”, which has been asked by philosophers from time immemorial, we must begin by pondering naive thoughts such as those above. The attitude of the scientist about such questions is quite ordinary; it is a simple extension of everyday logic.

We would try to touch a book if we thought its existence suspect. If we are still in doubt, we might ask someone nearby if he can see the book. If we try all the tests that are conceivable and still find no contradictions, we would conclude that the book is real. Of course, having once convinced ourselves that the book exists, we become unconscious of such questions.

But try to think of a situation when we encounter something completely new — say a UFO. How would we act? Then we would follow the steps for confirmation that were outlined above. The reality of the atom, in the final analysis, is established in the same way. If we suppose that the atom has such and such characteristics, and the atom passes all the tests we could perform to ascertain its characteristics, then we would come to believe in the existence of the atom. As we increase the kinds of tests to perform, our knowledge of the characteristics of the atoms becomes more and more accurate. We can also change or add to our theory of the atom without giving up the idea of the existence of the atom. If our theory were not a good one, then some inconsistencies would soon appear and we would have to do some unseemly patching up of the theory. If these patchings were needed one after the other, then our theory would probably be wrong and we would be better off scrapping the entire theory. On the other hand, if we had picked the correct theory, then we would be able to solve one mystery after another. This is very much like solving a crossword puzzle; we start out from what seem to be easy spots and try out some words. Even if we seem to succeed in some spots, many inconsistencies may crop up. We change our words a little but we still don’t succeed. Then suddenly we have an inspiration and the rest is almost automatic.

When a theory in physics reaches such a stage, we begin to believe in the theory as true and real. But since physics is not a closed subject but something that changes and grows, this kind of peaceful situation does not last very long. We reach a stage when there is a breach in the theoretical structure and the previous theory becomes useless. And here, we must repeat our labors once again; but we must also remember that the old theory that has withstood so many tests cannot be completely wrong. Since the old theory is not useful in the new situation, we must build a new structure that contains the old theory as something that is applicable only to special situations.

It is still true that the atoms are the chemical “elements”; but when I was a student it was believed that such things as the Yukawa mesons were the true elementary particles. Today even the general public knows of such terms as quarks and leptons. If you ask today’s physicist, he would say that these were the true “fundamental particles”.

What, then, are these quarks and leptons? What is their relationship with the atoms and mesons? It is the purpose of this book to answer these questions, and in doing so we will learn the history of the development of physics in this century.

But let us now stop telling the story in chronological order. Let us begin at the present and look back into the past.

2

QUARKS AND LEPTONS

The Quark — The Unconventional Elementary Particle that has not been Found

The oddly-named quark was thus christened by one of the originators of the quark theory, Murray Gell-Mann. I will speak of the origins of this particle in some detail later; the quark is one of the particles thought to be “elementary” at present, and it still has not escaped the realm of fiction completely.

As I have said before, the words “elementary particles” refer to the most fundamental constituents of matter. When I say that the quark has not escaped the realm of fiction, I am being very careful and conservative since the quark theory can explain all phenomena known at present. What I mean is that we have not reached the stage, as we have with protons and electrons for example, where we have absolutely no doubt about the existence of the quark.

Why is there this slight doubt? It is because, although the quark is supposed to have characteristics unseen in the known particles, no one has spotted anything that looks like it. It appears that one cannot take a quark out of matter and confirm its characteristics. On the contrary, other known particles can be taken out by themselves and measurements can be made on them. Nucleons and electrons were mere theoretical constructs based on conservation laws and so forth at first, but they were eventually isolated and and their masses and charges were measured.

Actually, the quark, if it existed by itself, should be very easy to identify. This is because the electric charge carried by the quark is supposed to be two-thirds or one-third of the unit charge, i.e., the charge carried by an electron or a proton. All elements known up to now, be they electrons or nuclei, have electric charges either zero or an integer multiple (±1, ±2, . . .) of the unit charge e carried by an electron. So no matter what piece of matter is examined, its total charge is always an integer multiple of e.

One other important thing is the law of conservation of charge. Although reactions occur among particles and one particle can change into another, or particles exchange their electric charges, the total number of charges never changes. This is a typical example of the conservation laws I mentioned above, and if this is true it is not hard to imagin...

Table of contents

• Cover
• Half title
• Title
• Copyright
• Preface
• Contents
• 1. What Is An Elementary Particle
• 2. Quarks And Leptons
• 3. Looking For Quarks
• 4. Various Accelerators
• 5. The Birth of The Yukawa Theory
• 6. The Apperance of New Particles
• 7. Orderliness of Elementary Particles And Conservation Laws
• 8. Parity And Conservation
• 9. Composite Models of Hadrons
• 10. The Quark Model
• 11. Evolution of The Quark Model
• 12. Charm And Its Followers
• 13. Quarks With Strings Attached
• 14. What Is a Parton?
• 15. Tomonaga’s Renormalization Theory
• 16. QCD — The Quantum Mechanics of Color
• 17. Spontaneous Breaking of Symmetry
• 18. The Leaning Structure of The Weak Interaction
• 19. The Weinberg-Salam Theory
• 20. The Unifield Field Theory
• 21. Program For Grand Unification
• Glossary
• Index