HIGGS
HIGGS
The Invention and Discovery of the ‘God Particle’
JIM BAGGOTT
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© Jim Baggott 2012
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First Edition published in 2012
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CONTENTS
About the Author
Preface
Foreword by Steven Weinberg
Prologue: Form and Substance
Part I: Invention
1 The Poetry of Logical Ideas
In which German mathematician Emmy Noether discovers the relationship between conservation laws and the deep symmetries of nature
2 Not a Sufficient Excuse
In which Chen Ning Yang and Robert Mills try to develop a quantum field theory of the strong nuclear force and annoy Wolfgang Pauli
3 People Will Be Very Stupid About It
In which Murray Gell-Mann discovers strangeness and the ‘Eightfold Way’, Sheldon Glashow applies Yang–Mills field theory to the weak nuclear force, and people are very stupid about it
4 Applying the Right Ideas to the Wrong Problem
In which Murray Gell-Mann and George Zweig invent quarks and Steven Weinberg and Abdus Salam use the Higgs mechanism to give mass to the W and Z particles (finally!)
5 I Can Do That
In which Gerard ’t Hooft proves that Yang–Mills field theories can be renormalized and Murray Gell-Mann and Harald Fritzsch develop a theory of the strong force based on quark colour
Part II: Discovery
6 Alternating Neutral Currents
In which protons and neutrons are shown to have an internal structure and the predicted neutral currents of the weak nuclear force are found, and then lost, and then found again
7 They Must Be Ws
In which quantum chromodynamics is formulated, the charmquark is discovered, and the W and Z particles are found, precisely where they were predicted to be
8 Throw Deep
In which Ronald Reagan throws his weight behind the Superconducting Supercollider, but when the project is cancelled by Congress six years later all that remains is a hole in Texas
9 A Fantastic Moment
In which the Higgs boson is explained in terms that a British politician can understand, hints of the Higgs are found at CERN, the Large Hadron Collider is switched on, and then blows up
10 The Shakespeare Question
In which the LHC performs better than anyone expected (except Lyn Evans), a year’s data is gathered in a few months and the Higgs boson runs out of places to hide
Epilogue: The Construction of Mass
Endnotes
Glossary
Bibliography
Index
ABOUT THE AUTHOR
Jim Baggott is an award-winning science writer. A former academic scientist, he now works as an independent business consultant but maintains a broad interest in science, philosophy, and history, and continues to write on these subjects in his spare time. His previous books have been widely acclaimed and include:
The Quantum Story: A History in 40 Moments (Oxford University Press, 2011);
Atomic: The First War of Physics and the Secret History of the Atom Bomb 1939–49 (Icon Books, 2009), shortlisted for the Duke of Westminster Medal for Military Literature, 2010;
A Beginner’s Guide to Reality (Penguin, 2005);
Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (Oxford University Press, 2004);
Perfect Symmetry: The Accidental Discovery of Buckminsterfullerene (Oxford University Press, 1994); and
The Meaning of Quantum Theory: A Guide for Students of Chemistry and Physics (Oxford University Press, 1992).
PREFACE
The news that something very much like the Higgs boson had been discovered, at CERN in Geneva on 4 July 2012, flashed instantaneously around the world like some highly contagious electronic virus. Headlines screamed of this latest triumph of high-energy physics. The discovery made front-page news, was featured in many evening news bulletins, and reached an audience of billions. Signals consistent with a particle that had first been hypothesized or ‘invented’ in 1964 had at last been found, 48 years later, at a cost of many billions of dollars.
So, what was all the fuss about? What is the Higgs boson and why does it matter so much? If this new particle really is the Higgs, what does it tell us about the material world and the evolution of the early universe? Was finding it really worth all the effort?
The answers to these questions can be found in the story of the so-called Standard Model of particle physics. As the name implies, this is the framework that physicists use to interpret the elementary constituents of all matter and the forces that bind matter together, or cause it to fall apart. It is a body of work built up over many decades of unstinting effort, which represents the physicists’ best efforts to interpret the physical world around us.
The Standard Model is not yet a ‘theory of everything’. It does not account for the force of gravity. In recent years you may have read about exotic new theories of physics which attempt to unify the fundamental forces, including gravity. These are theories such as supersymmetry and superstrings. Despite the efforts of hundreds of theorists engaged on these projects, these new theories remain speculative and have little or no supporting evidence from experiment. For the time being, and despite flaws that have been acknowledged since its inception in the late 1970s, the Standard Model is still where most of the real action is.
The Higgs boson is important in the Standard Model because it implies the existence of a Higgs field, an otherwise invisible field of energy which pervades the entire universe. Without the Higgs field, the elementary particles that make up you, me, and the visible universe would have no mass. Without the Higgs field mass could not be constructed and nothing could be.
It seems we owe quite a lot to the existence of this field. This is one of the reasons why the Higgs boson, the particle of the Higgs field, has been hyped in the popular press as the God particle. This is a name heartily despised by practising scientists, as
it overstates the importance of the particle and draws attention to the sometimes uneasy relationship between physics and theology. It is, however, a name much beloved by science journalists and popular science writers.
Many of the predicted consequences of the Higgs field were borne out in particle collider experiments in the early 1980s. But inferring the field is not the same as detecting its tell-tale field particle. It is therefore immensely reassuring to know that the field is very probably here, there, and everywhere. The possibility that the Higgs boson might not have been found was very real, and the implications for the Standard Model were potentially devastating.
I began writing this book in June 2010, two years before the discovery was made. I had just completed the manuscript of another book, called The Quantum Story: A History in 40 Moments which, as the title implies, is a history of quantum physics from 1900 to the present day. That book covered the development of the Standard Model and the invention of the Higgs field and its particle. A few months earlier CERN’s Large Hadron Collider reached record proton–proton collision energies of seven trillion electron volts, and I figured that a discovery might be possible within the next few years. Happily, I was proved right.
The Quantum Story was published in February 2011. The present book is based, in part, on that earlier work.
My thanks go to Latha Menon and the delegates at Oxford University Press, who were ready to risk commissioning a book about a particle that hadn’t yet been discovered. I have followed developments at CERN through the official channels, but acknowledge a debt to a number of high-energy physics bloggers, including Philip Gibbs, Tommaso Dorigo, Peter Woit, Adam Falkowski, Matt Strassler and Jon Butterworth. Thanks also to Jon Butterworth, Sophie Tesauri, James Gillies, Laurette Ponce, and Lyndon Evans for taking the time to talk to me and share their growing sense of excitement. I would also like to express my gratitude to Professors David Miller, and Peter Woit, who read and commented on the draft manuscript, and to Professor Steven Weinberg, who also read through the draft manuscript and kindly contributed a personal perspective in his Foreword. Be assured that the errors that remain are all my own work.
Jim Baggott
Reading, 6 July 2012.
FOREWORD
by Steven Weinberg
Many important scientific discoveries have been followed by popular books explaining these discoveries to general readers. But this is the first case I have seen of a book that has been largely written in anticipation of a discovery. The readiness of this book for publication immediately after the announcement in July 2012 of the discovery at CERN (with some corroboration from Fermilab) of a new particle that seems to be the Higgs particle testifies to the remarkable energy and enterprise of Jim Baggott and Oxford University Press.
The prompt publication of this book also testifies to the widespread public interest in this discovery. So it may be worthwhile if in this Foreword I add some remarks of my own about just what has been accomplished. It is often said that what was at stake in the search for the Higgs particle was the origin of mass. True enough, but this explanation needs some sharpening.
By the 1980s we had a good comprehensive theory of all observed elementary particles and the forces (other than gravitation) that they exert on one another. One of the essential elements of this theory is a symmetry, like a family relationship, between two of these forces, the electromagnetic force and the weak nuclear force. Electromagnetism is responsible for light; the weak nuclear force allows particles inside atomic nuclei to change their identity in radioactive decay processes. This symmetry brings the two forces together in a single ‘electroweak’ structure. The general features of the electroweak theory have been well tested; their validity is not what has been at stake in the recent experiments at CERN and Fermilab, and would not be seriously in doubt even if no Higgs particle had been discovered.
But one of the consequences of the electroweak symmetry is that, if nothing new is added to the theory, all elementary particles including electrons and quarks would be massless, which of course they are not. So, something has to be added to the electroweak theory, some new kind of matter or field, not yet observed in nature or in our laboratories. The search for the Higgs particle has been a search for the answer to the question: What is this new stuff we need?
The search for this new stuff has not been just a matter of noodling around at high energy accelerators, waiting to see what turns up. Somehow the electroweak symmetry, an exact property of the underlying equations of elementary particle physics, must be broken; it must not apply directly to the particles and forces we actually observe. It has been known since the work of Yoichiro Nambu and Jeffrey Goldstone in 1960–61 that symmetry-breaking of this sort is possible in various theories, but it had seemed that it would necessarily entail new massless particles, which were known not to exist.
It was the independent work of Robert Brout and François Englert; Peter Higgs; and Gerald Guralnik, Carl Hagen and Tom Kibble, all in 1964, that showed that in some kinds of theories these massless Nambu-Goldstone particles would disappear, serving only to give mass to force-carrying particles.* This is what happens in the theory of weak and electromagnetic forces proposed in 1967–68 by Abdus Salam and myself. But this still left open the question: What sort of new matter or field is actually breaking the electroweak symmetry?
There were two possibilities. One possibility was that there are hitherto unobserved fields that pervade empty space, and that just as the earth’s magnetic field distinguishes north from other directions, these new fields distinguish weak from electromagnetic forces, giving mass to the particles that carry the weak force and to other particles, but leaving photons (which carry the electromagnetic force) with zero mass. These are called ‘scalar’ fields, meaning that unlike magnetic fields they do not distinguish directions in ordinary space. Scalar fields of this general sort were introduced in the illustrative examples of symmetry-breaking used by Goldstone and later in the 1964 papers.
When Salam and I used this sort of symmetry-breaking in developing the modern ‘electroweak’ theory of weak and electromagnetic forces, we assumed that the symmetry breaking was due to fields of this scalar type, pervading all space. (A symmetry of this sort had already been hypothesized by Sheldon Glashow and by Salam and John Ward, but not as an exact property of the equations of the theory, so these theorists were not led to introduce scalar fields.)
One of the consequences of theories in which symmetries are broken by scalar fields, including the models considered by Goldstone and the 1964 papers and the electroweak theory of Salam and me, is that although some of these fields serve only to give mass to the force carrying particles, other scalar fields would be manifested in nature as new physical particles that could be created and observed in accelerators and particle colliders. Salam and I found we needed to put four scalar fields into our electroweak theory. Three of these scalar fields were used up in giving mass to the W+, W−, and Z0 particles – the ‘heavy photons’ – that in our theory carry the weak force (these particles were discovered at CERN in 1983–84, and found to have the masses predicted by the electroweak theory). One of the scalar fields was left over to be manifested as a physical particle, a bundle of the energy and momentum of this field. This is the ‘Higgs particle’ for which physicists have been searching for nearly thirty years.
But there was always a second possibility. There might instead be no new scalar fields pervading space, and no Higgs particle. Instead, the electroweak symmetry might be broken by strong forces, known as ‘technicolour forces’, acting on a new class of particles too heavy to have been observed yet. Something like this happens in superconductivity. This kind of theory of elementary particles was proposed in the late 1970s independently by Leonard Susskind and myself, and would lead to a whole forest of new particles, held together by technicolour forces. So this is the alternative with which we have been faced: Scalar fields? Or technicolour?
The discovery of the new particle casts a very st
rong vote in favour of the electroweak symmetry being broken by scalar fields, rather than by technicolour forces. This is why the discovery is important.
But much remains to be done to pin this down. The electroweak theory of 1967–68 predicted all of the properties of the Higgs particle, except its mass. With the mass now known experimentally, we can calculate the probabilities for all the various ways that Higgs particles can decay, and see if these predictions are borne out by further experiment. This will take a while.
The discovery of a new particle that appears to be the Higgs also leaves theorists with a difficult task, to understand its mass. The Higgs is the one elementary particle whose mass does not arise from the breakdown of the electroweak symmetry. As far as the underlying principles of the electroweak theory are concerned, the Higgs mass could have any value. That is why neither Salam nor I could predict it.
In fact, there is something puzzling about the Higgs mass we now do observe. It is generally known as the ‘hierarchy problem’. Since it is the Higgs mass that sets the scale for the masses of all other known elementary particles, one might guess that it should be similar to another mass that plays a fundamental role in physics, the so-called Planck mass, which is the fundamental unit of mass in the theory of gravitation (it is the mass of hypothetical particles whose gravitational attraction for each other would be as strong as the electric force between two electrons separated by the same distance). But the Planck mass is about a hundred thousand trillion times larger than the Higgs mass. So, although the Higgs particle is so heavy that a giant particle collider was needed to create it, we still have to ask, why is the Higgs mass so small?
____________
Jim Baggott suggested that I might add here some personal perspectives about the evolution of ideas in this field. I’ll mention just two points.
As Baggott describes in Chapter 4, Philip Anderson argued early, before 1964, that massless Nambu-Goldstone particles were not a necessary consequence of symmetry breaking. So why were I and other particle theorists not convinced by Anderson’s argument? It certainly did not reflect any judgment that Anderson did not have to be taken seriously. Of all the theorists who concerned themselves with condensed matter physics, no-one has seen more clearly than Anderson the importance of principles of symmetry, principles that have proved all-important in particle physics.
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