The Particle at the End of the Universe: How the Hunt for the Higgs Boson Leads Us to the Edge of a New World

by Sean Carroll

  THE PARTICLE AT THE END OF THE UNIVERSE

  How the Hunt for the Higgs Boson Leads Us to the Edge of a New World

  Sean Carroll

  DUTTON

  DUTTON

  Published by Penguin Group (USA) Inc.

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  Penguin Group (Canada), 90 Eglinton Avenue East, Suite 700, Toronto, Ontario M4P 2Y3, Canada (a division of Pearson Penguin Canada Inc.); Penguin Books Ltd, 80 Strand, London WC2R 0RL, England; Penguin Ireland, 25 St Stephen’s Green, Dublin 2, Ireland (a division of Penguin Books Ltd); Penguin Group (Australia), 707 Collins St., Melbourne, Victoria 3008, Australia (a division of Pearson Australia Group Pty Ltd); Penguin Books India Pvt Ltd, 11 Community Centre, Panchsheel Park, New Delhi – 110 017, India; Penguin Group (NZ), 67 Apollo Drive, Rosedale, Auckland 0632, New Zealand (a division of Pearson New Zealand Ltd); Penguin Books, Rosebank Office Park, 181 Jan Smuts Avenue, Parktown North 2193, South Africa; Penguin China, B7 Jaiming Center, 27 East Third Ring Road North, Chaoyang District, Beijing 100020, China

  Penguin Books Ltd, Registered Offices: 80 Strand, London WC2R 0RL, England

  Published by Dutton, a member of Penguin Group (USA) Inc.

  First printing, November 2012

  Copyright © 2012 by Sean Carroll

  All rights reserved. No part of this book may be reproduced, scanned, or distributed in any printed or electronic form without permission. Please do not participate in or encourage piracy of copyrighted materials in violation of the author’s rights. Purchase only authorized editions.

  REGISTERED TRADEMARK—MARCA REGISTRADA

  Library of Congress Cataloging-in-Publication Data has been applied for.

  ISBN 978-1-101-60970-5

  While the author has made every effort to provide accurate telephone numbers, Internet addresses, and other contact information at the time of publication, neither the publisher nor the author assumes any responsibility for errors, or for changes that occur after publication. Further, the publisher does not have any control over and does not assume any responsibility for author or third-party websites or their content.

  To Mom,

  who took me to the library

  People underestimate the impact of a new reality.

  —JOE INCANDELA, SPOKESPERSON FOR THE CMS COLLABORATION AT THE LARGE HADRON COLLIDER

  CONTENTS

  PROLOGUE

  ONE THE POINT

  TWO NEXT TO GODLINESS

  THREE ATOMS AND PARTICLES

  FOUR THE ACCELERATOR STORY

  FIVE THE LARGEST MACHINE EVER BUILT

  SIX WISDOM THROUGH SMASHING

  SEVEN PARTICLES IN THE WAVES

  EIGHT THROUGH A BROKEN MIRROR

  NINE BRINGING DOWN THE HOUSE

  TEN SPREADING THE WORD

  ELEVEN NOBEL DREAMS

  TWELVE BEYOND THIS HORIZON

  THIRTEEN MAKING IT WORTH DEFENDING

  APPENDIX ONE MASS AND SPIN

  APPENDIX TWO STANDARD MODEL PARTICLES

  APPENDIX THREE PARTICLES AND THEIR INTERACTIONS

  PHOTOGRAPHS

  FURTHER READING

  REFERENCES

  ACKNOWLEDGMENTS

  INDEX

  PROLOGUE

  JoAnne Hewett is feeling giddy, smiling broadly as she speaks enthusiastically into a video camera. An excited buzz filters up from partygoers at the Swiss consulate in San Francisco. It’s a unique event, celebrating the first protons circulating in the underground tunnel of the Large Hadron Collider (LHC) outside Geneva—an enormous particle accelerator on the French-Swiss border that has begun its quest to unlock the secrets of the universe. The champagne flows freely, and no wonder. Hewett’s voice rises with emphasis: “I’ve been waiting for this day for Twenty. Five. Years.”

  It’s a big moment. At this point in 2008, physicists have finally achieved what they have long insisted was necessary to make the next big step forward: a giant accelerator that would smash protons together at very high energies. For a while, they thought the United States was going to build such a machine, but things didn’t work out as anticipated. Hewett was just beginning graduate school in 1983, when the U.S. Congress first approved construction of the Superconducting Super Collider (SSC) in Texas. Slated to begin operation before the year 2000, it would have been the largest collider ever built. She, like so many of the brilliant and ambitious physicists of her generation, believed that discoveries there would form the foundation of their research careers.

  But the SSC was canceled, pulling the rug out from under physicists who had counted on it to shape the course of their field for decades to come. Politics and bureaucracy and infighting got in the way. Now the LHC, similar in many ways to what the SSC would have been, is at long last about to fire up for the first time, and Hewett and her colleagues are more than ready for it. “What I’ve done over the past twenty-five years is take every new crazy theory that anybody’s ever come up with and calculated its signature [how we identify new particles] at the SSC or LHC,” she says.

  There is another, more personal reason for Hewett’s giddiness. In the video, her red hair is very short, almost a crew cut. It’s not a fashion choice. Earlier in the year she was diagnosed with invasive breast cancer, with about a one-in-five chance that it would be terminal. She opted for an extremely aggressive treatment program, involving harsh chemotherapy and a seemingly endless series of surgeries. Her trademark red hair, usually reaching down to her waist, disappeared quickly. At times, she admits with a laugh, she kept her spirits up by thinking about what new particles would be found at the LHC.

  JoAnne and I have known each other for years, as friends and colleagues. My own expertise is primarily in cosmology, the study of the universe as a whole, which has recently enjoyed a golden age of new data and surprising discoveries. Particle physics, which has become inseparable from cosmology as an intellectual discipline, has nevertheless been starved for new experimental results that will upend the theoretical applecart and lead us forward to new ideas. The pressure has been building for a long time. Another physicist at the party, Gordon Watts of the University of Washington, was asked whether the long anticipation for the LHC has been stressful. “Oh yeah, completely. I have this shock of gray hair here now. My wife claims it’s because of our kid, but it’s really because of the LHC.”

  Particle physics stands on the brink of a new era, in which some theories are going to come crashing down, and perhaps others will turn out to be right on the money. Every physicist at the party has their favorite models—Higgs bosons, supersymmetry, technicolor, extra dimensions, dark matter—a tumble of exotic ideas and fantastic implications.

  “My hope for what the LHC will find is ‘none of the above,’” Hewett enthuses. “I honestly think it’s going to be a surprise, because I think nature is smarter than we are, and she’s got some surprises in store for us, and we’re going to have a hell of a fun time trying to figure it all out. And it’s going to be great!”

  That was 2008. In 2012, the San Francisco party to celebrate the inauguration of the LHC is over, and the era of discovery has been officially launched. Hewett’s hair has grown back. The treatments were agonizing, but they seem to have worked. And the experiment she’s been anticipating for her entire career is making history. After two and a half decades of theorizing, her ideas are finally being tested against real data—particles and interactions never seen before by human beings, surprises tha
t nature has been keeping hidden from us. Until now.

  Jump to July 4, 2012, opening day of the International Conference on High Energy Physics. It’s a biannual gathering, moving from city to city around the world, this year winding up in Melbourne, Australia. Hundreds of particle physicists, Hewett included, have filled the main auditorium to hear a special seminar. All the investment in the LHC, all the anticipation that has built up over the years, is about to pay off.

  The presentation itself is beamed to Melbourne from CERN, the laboratory in Geneva that is home to the LHC. There are two talks, which would ordinarily have been presented in Melbourne as part of the conference program. At the last minute, however, the powers that be decided that a moment of this magnitude should be shared with the many people who had helped make the LHC such a success. The sentiment was appreciated—hundreds of physicists at CERN have lined up for hours before the talks were scheduled to begin at nine a.m., Geneva time, camping out overnight with sleeping bags in hopes of getting a good seat.

  Rolf Heuer, director general of CERN, introduces the proceedings. There will be talks by American physicist Joe Incandela and Italian physicist Fabiola Gianotti, the spokespersons for the two major experiments that work to collect and analyze LHC data. Both experiments include more than three thousand collaborators each, most of whom are glued to computer monitors scattered around the globe. The event is being live-streamed, not only to Melbourne, but to anyone who wants to hear the results in real-time. It’s an appropriate medium for this celebration of modern Big Science—a high-tech international effort with big stakes and exhilarating rewards.

  Traces of nervous energy are evident in both Gianotti’s and Incandela’s talks, but the presentations speak for themselves. They each give heartfelt thanks to the many engineers and scientists who helped make the experiments possible. Then they carefully explain why we should all believe the results they are about to present, demonstrating that they understand how their machines are working and that the analysis of the data is precise and reliable. Only after the stage has been immaculately set do they show us what they’ve found.

  And there it is. A handful of graphs that wouldn’t seem like much to the untutored eye, but with a consistent feature: more events (collections of particles streaming from a single collision) than expected with a certain particular energy. All the physicists in the audience know immediately what it means: a new particle. The LHC has glimpsed a part of nature that had heretofore never been seen. Incandela and Gianotti go through the painstaking statistical analysis meant to separate true discoveries from unfortunate statistical fluctuations, and the results in both cases speak without ambiguity: This is something real.

  Applause. In Geneva, Melbourne, and around the world. The data are so precise and clear that even scientists who had worked on the experiments for years are taken aback. Welsh physicist Lyn Evans, who more than anyone else was responsible for guiding the LHC through its rocky path to completion, declared himself “gobsmacked” at the exquisite agreement between the two experiments.

  I was at CERN myself that day, masquerading as a journalist in a pressroom next to the main auditorium. Journalists aren’t supposed to clap at the news events they cover, but the assembled reporters gave in to the overwhelming emotion of the moment. This wasn’t just a success for CERN, or for physics; this was a success for the human race.

  We think we know what’s been found: an elementary particle called the “Higgs boson,” after Scottish physicist Peter Higgs. Higgs himself was in the room for the seminars, eighty-three years old and visibly moved: “I never thought I’d see this happen in my lifetime.” Several other senior physicists who had likewise proposed the same idea back in 1964 were also present; the conventions by which theories are named aren’t always fair, but this was a moment when everyone could join in the celebration.

  So what is the Higgs boson? It’s a fundamental particle of nature, of which there aren’t many, and a very special kind of particle to boot. Modern particle physics knows of three kinds of particles. There are particles of matter, like electrons and quarks, that constitute the atoms that make up everything we see. There are the force particles that carry gravity and electromagnetism and the nuclear forces, which hold the matter particles together. And then there is the Higgs, in its own unique category.

  The Higgs is important not for what it is but for what it does. The Higgs particle arises from a field pervading space, known as the “Higgs field.” Everything in the known universe, as it travels through space, moves through the Higgs field; it’s always there, lurking invisibly in the background. And it matters: Without the Higgs, electrons and quarks would be massless, just like photons, the particles of light. They would move at the speed of light themselves, and it would be impossible to form atoms and molecules, much less life as we know it. The Higgs field isn’t an active player in the dynamics of ordinary matter, but its presence in the background is crucial. Without it, the world would be an utterly different place. And now we’ve found it.

  Some words of caution are in order. What we actually have in hand is evidence for a very Higgs-like particle. It has the right mass, it is produced and decays in roughly the expected ways. But it’s too early in the game to say for sure that what we’ve discovered is definitely the simple Higgs predicted by the original models. It could be something more complicated, or be part of an elaborate web of related particles. But we’ve definitely found some new particle, and it acts like we think a Higgs boson should. For the purposes of this book, I’m going to treat July 4, 2012, as the day the discovery of the Higgs boson was announced. If reality turns out to be more subtle, then all the better for everyone—physicists live for surprises.

  Hopes are high that the Higgs discovery represents the beginning of a new age in particle physics. We know that there is more to physics than we currently understand; studying the Higgs offers a new window into worlds yet unseen. Experimenters like Gianotti and Incandela have a new specimen to study; theorists like Hewett have new clues to build better models. Our understanding of the universe has taken a huge, long-anticipated step forward.

  This is the story of the people who have devoted their lives to discovering the ultimate nature of reality, of which the Higgs is a crucial component. There are theorists, sitting with pencil and paper, fueled by espresso and heated disputes with colleagues, turning over abstract ideas in their minds. There are engineers, pushing machines and electronics well beyond the limits of existing technology. And most of all there are the experimenters, bringing the machines and the ideas together to discover something new about nature. Modern physics at the cutting edge involves projects that cost billions of dollars and take decades to complete, requiring extraordinary devotion and a willingness to bet high stakes in search of unique rewards. When it all comes together, the world changes.

  Life is good. Have another glass of champagne.

  ONE

  THE POINT

  In which we ask why a group of talented and dedicated people would devote their lives to the pursuit of things too small to be seen.

  Particle physics is a curious activity. Thousands of people spend billions of dollars building giant machines miles across, whipping around subatomic particles at close to the speed of light and crashing them together, all to discover and study other subatomic particles that have essentially no impact on the daily lives of anyone who is not a particle physicist.

  That’s one way of looking at it, anyway. Here’s another way: Particle physics is the purest manifestation of human curiosity about the world in which we live. Human beings have always asked questions, and since the ancient Greeks more than two millennia ago, the impulse to explore has grown into a systematic, worldwide effort to discover the basic rules governing how the universe works. Particle physics arises directly from our restless desire to understand our world; it’s not the particles that motivate us, it’s our human desire to figure out what we don’t understand.

  The early years of the twenty-fi
rst century are a turning point. The last truly surprising experimental result to emerge from a particle accelerator was in the 1970s, more than thirty-five years ago. (The precise date would depend on your definition of “surprising.”) It’s not because the experimentalists have been asleep at the switch—far from it. The machines have improved by leaps and bounds, reaching into realms that seemed impossibly far away just a short time ago. The problem is that they haven’t seen anything we didn’t already expect them to see. For scientists, who are always hoping to be surprised, that’s extremely annoying.

  The problem, in other words, isn’t that the experiments have been inadequate—it’s that the theory has been too good. In the specialized world of modern science, the roles of “experimentalists” and “theorists” have become quite distinct, especially in particle physics. Gone are the days—as recent as the first half of the twentieth century—when a genius like Italian physicist Enrico Fermi could propose a new theory of the weak interactions, then turn around and guide the construction of the first self-sustained artificial nuclear chain reaction. Today, particle theorists scribble equations on blackboards, which ultimately become specific models, which are tested by experimentalists who gather data from exquisitely precise machines. The best theorists keep close tabs on experiments and vice versa, but no one person is a master of both.

  The 1970s saw the finishing touches put on our best theory of particle physics, which goes by the fantastically uninspiring name of the “Standard Model.” It’s the Standard Model that describes quarks, gluons, neutrinos, and all the other elementary particles you may have heard of. Like Hollywood celebrities or charismatic politicians, scientific theories are put on a pedestal just so we can tear them down. You don’t become a famous physicist by showing that someone else’s theory is right; you become famous by showing where someone else’s theory goes wrong, or by proposing a better theory.