Hidden Worlds

by Smith, Timothy Paul

• ISBN13: 9780691057736
• ISBN10: 0691057737
• Format: Hardcover
• Copyright: 2002-11-01
• Publisher: Princeton Univ Pr

Summary

No one has ever seen a quark. Yet physicists seem to know quite a lot about the properties and behavior of these ubiquitous elementary particles. Here a top researcher introduces us to a fascinating but invisible realm that is part of our everyday life. Timothy Smith tells us what we know about quarks--and how we know it.Though the quarks that make science headlines are typically laboratory creations generated under extreme conditions, most quarks occur naturally. They reside in the protons and neutrons that make up almost all of the universe's known matter, from human DNA to distant nebulae, from books and tables to neutron stars. Smith explains what these quarks are, how they act, and why physicists believe in them sight unseen. How do quarks arrange themselves? What other combinations can nature make? How do quarks hold nuclei together? What else is happening in their hidden worlds? It turns out that these questions can be answered using a few simple principles, such as the old standby: opposites attract. With these few principles, Smith shows how quarks dance around each other and explains what physicists mean when they refer to "up" and "down" quarks and talk about a quark's color, flavor, and spin.Smith also explains how we know what we know about these oddly aloof particles, which are eternally confined inside larger particles. He explains how quark experiments are mounted and how massive accelerators, targets, and detectors work together to collect the data that scientists use to infer what quarks are up to.A nonmathematical tour of the quark world, this book is written for students, educators, and all who enjoy scientific exploration--whether they seek a taste of subnuclear physics or just wonder about nature on the smallest of scales.

Author Biography

Timothy Paul Smith is a Research Scientist at the Massachusetts Institute of Technology Bates Linear Accelerator Center and Research Professor at Dartmouth College.

Table of Contents

List of Figures	ix
Acknowledgments	xi
Hidden Worlds: The Search for Quarks in Ordinary Matter	1	(14)
The Rise and Fall (for the right reasons) and Rise Again of the Quark Hypothesis	15	(18)
The Players and the Stage	33	(18)
The Nature of the Evidence	51	(17)
Measuring a Rainbow	68	(22)
Particle Taxonomy and Quark Soup	90	(19)
The Shape of Things	109	(22)
Three Quarks Plus	131	(17)
Digging a Little Deeper	148	(13)
A New Age of Exploration within the Hidden World	161	(4)
Glossary	165	(10)
Index	175

Excerpts

HIDDEN WORLDS: THE SEARCH FOR QUARKS IN ORDINARY MATTER

BY MOST accounts, the quest to understand the basic structure of matter has been an old-fashioned success story of growth and expansion. Machines have become bigger; computers have gotten faster. Beams of light or particles have become brighter and more powerful. Interactions of elementary particles have become more fleeting, and have given rise to ever more energetic and more exotic by-products. The basic engine driving this growth, the particle accelerator, began as a tabletop instrument you could hold in your hand-and was no more powerful than a lightbulb. By the 1950s, accelerators had grown large enough to fill a small warehouse, and drew enough power to run a large printing press. Now they need farmland or rangeland to accommodate their dimensions, and enough power to run a medium-size city.

Higher energies enable experimenters to "see" finer and finer details, to probe and analyze matter at smaller and smaller scales. Today the world's most powerful particle accelerators are operated at FermiLab, on the Illinois prairie about an hour's drive southwest of Chicago, and at CERN (Council Europe ´en pour la Recherche Nucle ´aire-The European Organization for Nuclear Research), the particle physics laboratory, in a rural suburb of Geneva just under the Swiss border with France. The FermiLab accelerator is a ring 4 miles around; inside the ring there is plenty of room to pasture a herd of buffalo. At CERN, an even larger accelerator is under construction. When it comes into service in 2006, it will be 16 miles (27 kilometers) around.

At both these laboratories the combined accelerators and detectors are, in effect, magnificent microscopes that owe their magnifying powers to their ability to focus energy into electrons or protons that carry a trillion volts. With these energies both machines can resolve details in the structure of matter smaller than 10-18 meter across-a billionth of a billionth of a meter. And both machines are examples of science so big and so costly that they stretch the resources of individual sovereign states. CERN is funded by a European-wide consortium, and both laboratories are used by a collaboration of scientists from all over the world.

But the high price of the ability to probe such details is not measured only in dollars and cents. High energies can magnify, but they also carry great destructive power. It was often said in the early decades of high-energy physics that its basic investigative tactic was much like smashing two fine Swiss watches together in midair and then trying to understand how they worked by looking at the fragments. In fact, the true situation is even worse than that. Particles accelerated by today's state-of-the-art machines collide so violently that the collision fragments are often strikingly different from ordinary matter.

For many-even most-elementary particle physicists working today, that's just the point. The emphasis in the past few decades at such places as FermiLab and CERN has been to produce some of the most exotic particles predicted by theory, the "top" and the "bottom" mesons. On a different front, at Brookhaven National Laboratory on Long Island, a campaign is under way to create a "quark-gluon plasma," a state of matter, as some physicists have described it, "not seen since the big bang." Such phenomena can be studied only by accelerating, smashing, and, in effect, heating and squeezing ordinary matter to conditions beyond the edge of extreme: far beyond the temperatures and pressures prevailing even in the cores of the hottest stars, to regimes in which matter takes on strange and outlandish forms that do not exist at all in the universe as we now know it.

Yet in that rush to re-create such exotic conditions, and to study their implications for the birth, death, and ultimate structure of the universe, elementary particle physicists have almost forgotten the world in which we live. If the initial intellectual impulse was to probe the proton and the neutron in order to understand their role in ordinary matter, that impulse has virtually disappeared from the CERNs, FermiLabs, and Brookhavens of the world.

To my mind, that's a shame. I don't live at the dawn of time, and I don't live in a fantastically hot and energetic collision. I live in a world made up primarily of electrons, neutrons, and protons. And I want to know how they act and interact under "ordinary" conditions. I am a nuclear physicist, or to be more precise, a nucleon physicist. "Nucleon" is the generic term for neutron or proton, the particles that make up the atomic nucleus. My work and the work of my closest colleagues is dedicated to understanding the physics of ordinary nucleons, a layer in the onionlike organization of matter that gives rise to an incredibly rich set of phenomena. Those phenomena are quite literally destroyed among the debris of the highest-energy accelerators.

ORDINARY MATTER

Think about it this way: the universe that we understand is more than 99.95 percent neutrons and protons by mass. It is true that there are things we physicists don't understand, such as the stuff astronomers and cosmologists call dark matter. But the range of things we do understand in terms of electrons, neutrons, and protons is astonishing. Stars, those hot, glowing beacons in space, those light- and life-giving orbs suspended in the void, are made up of these three materials. Nebulae, the misty veils of interstellar gas and dust that stretch like curtains across the galaxy, are made of the same stuff. Even the most exotic of stars, the so-called neutron stars, are made of the same three ingredients.

Closer to home, the wind, the rain, and the earth beneath our feet are made of these three building blocks. Even the substance of life itself-the blood coursing through our veins, the brain and nerve tissue that provide the scaffolding for our thoughts, the deoxyribonucleic acid (DNA) that carries the blueprints from which each of us is built-is made up entirely of electrons, neutrons, and protons. Books and tables, hands, hearts, and heads, are made of these three most basic substances.

At one time, not so many years ago, the study of such particles coincided with the frontiers of high-energy physics. One of the first laboratories I ever worked with was the National Instituut voor Kernfysica en Hoge Energie Fysica (NIKHEF) in Amsterdam, The Netherlands. Even if you don't speak Dutch, you can probably understand most of the name: read "hoge energie fysica" phonetically and you will hear "high-energy physics." Not so obvious is "kern-fysica," which corresponds to the English phrase "nuclear physics." Still, whether one uses the term "nuclear" or "kernel," the word is meant to emphasize the role of the nucleus at the very heart of the atom.

That's an important clue to understanding what stirs the soul of the nuclear and the high-energy physicist. When the word "nucleus" was coined in 1912, it was viewed as the "atom" had been before it: as the ultimate, indivisible, fundamental particle of matter. The nucleus stood at the core or kernel of the atom, the sun about which the planetary electrons orbited. Nuclear physics, therefore, was essentially a quest to discover and describe the most basic building blocks of the universe. When physicists discovered that the nucleus itself was divisible into nucleons and had structure, and that those nucleons also had internal structure, the "dream of a final theory" of ultimate particles had to be abandoned within the domain of nuclear physics. That dream was passed on to high-energy physics, a discipline that, by its very name, no longer defined itself by assumptions about where the ultimate particles would be found.

The NIKHEF accelerator could accelerate electrons to energies of 770 million electron volts (MeV). A 770 MeV electron beam can probe matter at a scale slightly smaller than a fermi, or 10-15 meter. That is just powerful enough to resolve structure in the atomic nucleus itself; you could say that NIKHEF marks the start of nucleon physics.

Another laboratory that I visited recently is called DESY (pronounced "daisy"), the Deutsches Elektronen-Synchrotron, in Hamburg, Germany. DESY's beam energy is 30 billion electron volts (Giga electron volts [GeV]), 40 times more energetic than NIKHEF's, which also makes its resolution 40 times finer. The trade-off is that DESY's "field of view" is too small to be of much use for the study of nucleons. It could reasonably be argued that DESY marks the energetic upper limit of nucleon physics. The beam energy is so high that what you see are the "bare" constituents of nucleons. These constituents interact so violently that the nucleons themselves no longer maintain their identities, but become transformed instead into new and exotic particles. Indeed, most physicists at DESY identify themselves as high-energy physicists, not nuclear or nucleon physicists at all.

Beyond DESY, the high-energy frontier has moved even farther on, to CERN, to FermiLab, to Brookhaven, and elsewhere. NIKHEF and other accelerators of its class have long since been displaced as record holders for high energy, just as they had displaced their predecessors. But the continual historical advance of the high-energy frontier hardly means that the "captured" territories have been subdued, much less fully mapped or colonized. The physicists rushing ahead with plans for ever more powerful accelerators, for probing ever more deeply into the ultimate building blocks of matter, have seldom stopped to fully plumb the structures that they found along the way. Yet nucleons represent a level in the organization of matter having exceptional stability, unique in the universe. Many physicists, and I am one of them, want to know as much as possible about electrons, neutrons, and protons.

WHAT MAKES A NUCLEON?

Just what do we physicists already know about these three particles? In the case of the electron, the summary can be quite brief. Electrons stand stark and innocent before us. What we see is essentially what we get: infinitesimally small particles, each with one unit of negative electric charge (-1), a spin of 1/2, and a mass of 9 x 10-31 kilogram-some 2,000 times lighter than either the proton or the neutron.

And that is all. We can say no more about the physical properties of the electron because it appears that there is nothing more to say. I don't mean to be dismissive about the electron or its physics. Electrons are undoubtedly useful to twenty-first-century humankind, when we push or pull them through wires in the form of electricity. They are fundamental, of course, to the architecture of matter: the number of electrons and their orbital patterns in an atom or a molecule are what give rise to all of chemistry. Finally, the self-interaction of the electron is at the core of one of the greatest theoretical successes of the twentieth century: quantum electrodynamics, or QED. QED serves as a prototype on which theories of other particles (the quarks) are modeled. And QED has generated some of the most precise, experimentally confirmed predictions in all of physics.

But the electron itself appears to be truly elementary. Electrons seem to have persisted unchanged since the dawn of time, and they are likely to remain as they are, immortal, until the final sunset of the universe. There is no evidence that anything exists inside the electron; there are no "ultimate electrons" rattling around inside its shell. In all experiments ever performed it really and truly appears to be pointlike.

Not so the nucleons: the proton and the neutron. A hundred thousand times smaller than the smallest atom, both the proton and the neutron are measured in fermis. Yet as small as that might be, there is a world hidden inside each one. At first glance that world appears to be exceedingly simple. Protons and neutrons are each made up of exactly three particles known as quarks. Whatever quarks really are, just think of them for the moment as three balls rattling around inside of the sphere we call a proton or a neutron. There are two common kinds of quark, known as the up quark and the down quark. The proton is made up of two up quarks and one down quark. The neutron reverses those numbers: it is made up of one up quark and two down quarks. The fact that the proton and the neutron each have three quarks gives rise to striking similarities in their masses, sizes, and interactions. But that single difference, an extra up quark in the proton, an extra down quark in the neutron, also accounts for their unique characteristics: their differing electric charge, their decay patterns, and the details of how they couple.

The genesis of such profound differences merely out of varying combinations of simple parts should not be too surprising. Science has vast experience at larger scales with objects whose distinctive properties arise out of the number, identity, and arrangement of their parts. The familiar shorthand for specifying molecules takes tacit advantage of the fact that it is often enough just to enumerate their elemental components: H2O-water-is two parts hydrogen and one part oxygen. Of course there are cases in which one set of atomic ingredients (such as C6H12O6) can give rise to two or more distinct but related molecules (in this case, glucose and fructose). Chemists call them isomers.

When we ratchet up the magnification and view the world on the atomic level, the very identity of an atom-the definition of its elemental type-depends solely on the number of protons in its nucleus (which is equal to the number of electrons orbiting that nucleus, in the electrically neutral atom). Helium is helium because it has two protons and two electrons. Carbon is carbon because it has six protons and six electrons. Quantum mechanics dictates how those six electrons in the carbon atom arrange themselves physically, and that arrangement in turn ordains the chemistry of carbon: what it will bind to, with what strength, in what configurations. The concept of isomer has an analogue in atomic physics as well: a given element can come in several forms known as isotopes, chemically almost indistinguishable from one another, but different nonetheless in the number of neutrons that share real estate in the nucleus with the protons.

In sum, it seems entirely natural that the properties of the proton and the neutron themselves arise from the number and arrangement of the quarks that make them up. One of the chief burdens of this book is to show how their properties and configurations give rise to a "quark chemistry" of remarkable complexity-just as the configurations of atoms in molecules and the configurations of electrons in atoms give rise to ordinary chemistry.

Continues...

Excerpted from Hidden Worlds by Timothy Paul Smith Copyright © 2002 by Princeton University Press
Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.