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

  But the Standard Model is stubborn. For decades now, every experiment that we can do here on earth has duly confirmed its predictions. An entire generation of particle physicists has risen up the academic ladder from students to senior professors, all without having a single new phenomenon that they could discover or explain. The anticipation has been close to unbearable.

  All this is changing. The Large Hadron Collider represents a new era in physics, smashing together particles with an energy never before achieved by humankind. And it’s not just higher energy. It’s an energy we’ve been dreaming about for years, in which we expect to find new theoretically predicted particles and hopefully some surprises—the energy where the force known as the “weak interaction” hides its secrets.

  The stakes are high. Peering into the unknown for the first time, anything could happen. There are scads of competing theoretical models hoping to anticipate what the LHC will find. You don’t know what you’re going to see until you look. At the center of the speculation lies the Higgs boson, an unassuming particle that represents both the last piece of the Standard Model, and the first glimpse into the world beyond.

  A big universe made of little pieces

  Near the Pacific coast in Southern California, about an hour-and-a-half drive south of where I live in Los Angeles, there is a magical place where dreams come to life: Legoland. At Dino Island, Fun Town, and other attractions, children marvel at an elaborate world constructed from Legos, tiny plastic blocks that can be fitted together in limitless combinations.

  Legoland is a lot like the real world. At any moment, your immediate environment typically contains all sorts of substances: wood, plastic, fabric, glass, metal, air, water, living bodies. Very different kinds of things, with very different properties. But when you look more closely, you discover that these substances aren’t truly distinct from one another. They are simply different arrangements of a small number of fundamental building blocks. These building blocks are the elementary particles. Like the buildings in Legoland, tables and cars and trees and people represent some of the amazing diversity you can achieve by starting with a small number of simple pieces and fitting them together in a variety of ways. An atom is about one-trillionth the size of a Lego block, but the principles are similar.

  We take for granted the idea that matter is made of atoms. It’s something we’re taught in school, where we do chemistry experiments in classrooms with the periodic table of the elements hanging on the wall. It’s easy to lose sight of how amazing that fact is. Some things are hard, some are soft; some things are light, some are heavy; some things are liquid, some are solid, some are gas; some things are transparent, some are opaque; some things are alive, some are not. But beneath the surface, all these things are really the same kind of stuff. There are about one hundred atoms listed in the periodic table, and everything around us is just some combination of those atoms.

  The hope that we can understand the world in terms of a few basic ingredients is an old idea. In ancient times, a number of different cultures—Babylonians, Greeks, Hindus, and others—invented a remarkably consistent set of five “elements” out of which everything else was made. The ones we are most familiar with are earth, air, fire, and water, but there was also a heavenly fifth element of aether, or quintessence. (Yes, that’s where the movie with Bruce Willis and Milla Jovovich got its name.) Like many ideas, this one was developed into an elaborate system by Aristotle. He suggested that each element sought a particular natural state; for example, earth tends to fall and air tends to rise. By mixing the elements in different combinations, we could account for the different substances we see around us.

  Democritus, a Greek philosopher who predated Aristotle, originally suggested that everything we know is made of certain tiny indivisible pieces, which he called “atoms.” It’s an unfortunate accident of history that this terminology was seized upon by John Dalton, a chemist who worked in the early 1800s, to refer to the pieces that define chemical elements. What we now think of as an atom is not indivisible at all—it consists of a nucleus made of protons and neutrons, around which orbit a collection of electrons. Even the protons and neutrons aren’t indivisible; they are made of smaller pieces called “quarks.”

  The quarks and electrons are the real atoms, in Democritus’s sense of indivisible building blocks of matter. Today we call them “elementary particles.” Two kinds of quarks—known playfully as “up” and “down”—go into making the protons and neutrons of an atomic nucleus. So, all told, we need only three elementary particles to make up every single piece of matter that we immediately perceive in the environment around us—electrons, up quarks, and down quarks. That’s an improvement over the five elements of antiquity, and a big improvement over the periodic table.

  Boiling the world down to just three particles is a bit of an exaggeration, however. While electrons and up and down quarks are enough to account for cars and rivers and puppies, they aren’t the only particles we’ve discovered. There are actually twelve different kinds of matter particles: six quarks that interact strongly and get confined inside larger collections like protons and neutrons, and six “leptons” that can travel individually through space. We also have force-carrying particles that hold them together in the different combinations we see. Without force particles, the world would be a boring place indeed—individual particles would just move on straight lines through space, never interacting with one another. It’s a fairly small set of ingredients to explain everything we see around us, but frankly, it could be simpler. Modern particle physicists are driven by a desire to do better.

  The Higgs boson

  That’s the Standard Model of particle physics: twelve matter particles, plus a group of force-carrying particles to hold them all together. Not the tidiest picture in the world, but it fits all the data. We have assembled all the pieces needed to successfully describe the world around us, at least here on earth. Out in space we find evidence for things like dark matter and dark energy, stubborn reminders that we certainly don’t understand everything yet—these are most certainly not explained by the Standard Model.

  For the most part the Standard Model divides nicely into matter particles and force-carrying particles. The Higgs boson is different. Named after Peter Higgs, who was one of several people who proposed the idea back in the 1960s, the Higgs boson is somewhat of an ugly duckling. Technically speaking it’s a force-carrying particle, but it’s a different kind of force carrier from the ones we’re most familiar with. From the viewpoint of a theoretical physicist the Higgs seems like an arbitrary and whimsical addition to an otherwise beautiful structure. If it weren’t for the Higgs boson, the Standard Model would be the epitome of elegance and virtue; as it is, it’s a bit of a mess. And finding the mess-maker has proven to be quite a challenge.

  So why were so many physicists convinced that the Higgs boson must exist? You’ll hear explanations like “to give mass to other particles” and “to break symmetries,” both of which are true but not easy to absorb at first glance. The main point is that without the Higgs boson, the Standard Model would look very different, and not at all like the real world. With the Higgs boson, it’s a perfect match.

  Theoretical physicists certainly tried their best to come up with theories that didn’t have a Higgs boson, or one in which the boson was quite different from the standard story. Many of the theories failed when confronted with the data, and others seemed unnecessarily complicated. None looked like a true upgrade.

  And now we’ve found it. Or something very much like it. Depending on how careful physicists are being, they will say either, “We’ve discovered the Higgs boson,” or “We’ve discovered a Higgs-like particle,” or even “we’ve discovered a particle that resembles the Higgs.” The July 4 announcement described a particle that behaves very much like the Higgs is supposed to behave—it decays into certain other particles in more or less the ways we expect it to. But it’s still early, and as we collect more data there is plenty of room
for surprises. Physicists don’t want it to be the Higgs we all expect; it’s always more interesting and fun to find something unexpected. There are already tiny hints in the data that this new particle might not be exactly the Higgs we expect. Only further experiments will reveal the truth.

  Why we care

  I was once interviewed by a local radio station about particle physics, gravitation, cosmology, things like that. It was 2005, the centenary of Albert Einstein’s “miraculous year” of 1905, in which he published a handful of papers that turned the world of physics on its head. I did my best to explain some of these abstract concepts, waving my hands up and down, which I can’t help but do even when I know I’m on the radio.

  The interviewer seemed happy, but after we finished and he was packing up his recording gear, a lightbulb went off in his head. He asked if I would answer one more question. I said sure, and he once again deployed his microphone and headphones. The question was simple: “Why should anybody care?” None of this research is going to lead to a cure for cancer or a cheaper smartphone, after all.

  The answer I came up with still makes sense to me: “When you’re six years old, everyone asks these questions. Why is the sky blue? Why do things fall down? Why are some things hot and others cold? How does it all work?” We don’t have to learn how to become interested in science—children are natural scientists. That innate curiosity is beaten out of us by years of schooling and the pressures of real life. We start caring about how to get a job, meet someone special, raise our own kids. We stop asking how the world works, and start asking how we can make it work for us. Later I found studies showing that kids love science up until the ages of ten to fourteen years old.

  These days, after pursuing science seriously for more than four hundred years, we actually have quite a few answers to offer the six-year-old inside each of us. We know so much about the physical world that the unanswered questions are to be found in remote places and extreme environments. That’s physics, anyway; in fields like biology or neuroscience, we have no difficulty at all asking questions to which the answers are still elusive. But physics—at least the subfield of “elementary” physics, which looks for the basic building blocks of reality—has pushed the boundaries of understanding so far that we need to build giant accelerators and telescopes just to gather new data that won’t fit into our current theories.

  Over and over again in the history of science, basic research—pursued just for the sake of curiosity, not for any immediate tangible benefit—has proven, almost despite itself, to lead to enormous tangible benefits. Way back in 1831, Michael Faraday, one of the founders of our modern understanding of electromagnetism, was asked by an inquiring politician about the usefulness of this newfangled “electricity” stuff. His apocryphal reply: “I know not, but I wager that one day your government will tax it.” (Evidence for this exchange is sketchy, but it’s a sufficiently good story that people keep repeating it.) A century later, some of the greatest minds in science were struggling with the new field of quantum mechanics, driven by a few puzzling experimental results that ended up overthrowing the basic foundations of all of physics. It was fairly abstract at the time, but subsequently led to transistors, lasers, superconductivity, light-emitting diodes, and everything we know about nuclear power (and nuclear weapons). Without this basic research, our world today would look like a completely different place.

  Even general relativity, Einstein’s brilliant theory of space and time, turns out to have down-to-earth applications. If you’ve ever used a global positioning system (GPS) device to find directions somewhere, you’ve made use of general relativity. A GPS unit, which you might find in your cell phone or car navigation system, takes signals from a series of orbiting satellites and uses the precise timing of those signals to triangulate its way to a location here on the ground. But according to Einstein, clocks in orbit (and therefore in a weaker gravitational field) tick just a bit faster than those at sea level. A small effect, to be sure, but it builds up. If relativity weren’t taken into account, GPS signals would gradually drift away from being useful—over the course of just one day, your location would be off by a few miles.

  But technological applications, while important, are ultimately not the point for me or JoAnne Hewett or any of the experimentalists who spend long hours building equipment and sifting through data. They’re great when they happen, and we won’t turn up our noses if someone uses the Higgs boson to find a cure for aging. But it’s not why we are looking for it. We’re looking because we are curious. The Higgs is the final piece to a puzzle we’ve been working on solving for an awful long time. Finding it is its own reward.

  The Large Hadron Collider

  We wouldn’t have found the Higgs without the Large Hadron Collider—another dreary name for an inspiring embodiment of the human passion for discovery. The LHC is the largest, most complex machine ever built by human beings, and it came in at a cool nine billion dollars. The scientists who work at CERN hope it will run productively for fifty years. But they aren’t that patient; it would be nice to get some world-changing discoveries right away, thank you very much.

  The LHC is gargantuan in every way it can be measured. It was first dreamed up in the 1980s, and approval to start building was given in 1994. Well before it was turned on, the LHC had made big news, as lawsuits attempted to halt its construction on the grounds that it might produce world-consuming black holes. Those were successfully squashed, and the giant collider went to work in earnest in 2009.

  Around the world on December 13, 2011, physicists—and quite a few interested onlookers—huddled in seminar rooms and around computer terminals to listen to two talks by researchers from the LHC. The subject was the search for the Higgs boson. This kind of topic is a very frequent subject for physics seminars, and the message is almost always “The search is going well! Wish us luck!” This time was different. Rumors had sped around the Internet for several days before, hinting that we weren’t just going to get the usual message—this time, they would be saying, “Okay, we might actually be seeing something. Maybe we’ve finally found evidence that the Higgs boson is really there.”

  The answer is yes, there were hints that the LHC was actually seeing the Higgs. Just hints, mind you; not the final word. The LHC smashed protons together at unbelievable energies, and two giant experimental detectors looked at what particles emerged from those collisions; the number of times that two high-energy photons (particles of light) were produced at a certain energy was just a smidgen bigger than we would have expected if there were no Higgs boson. Evidence that something was likely going on, to be sure, but not yet a discovery. But everything smelled right. Rolf Heuer ended the press conference with a flourish: “See you next year with a discovery.”

  And so they did. On July 4, 2012, two more seminars brought us an update on the search for the Higgs. This time it wasn’t a matter of tantalizing hints; they had found a new particle, without question. Thousands of physicists around the world clapped with joy but also exhaled with relief; the LHC was a success.

  Crossroads

  Particle physics stands at a critical threshold. It’s a foundational part of the human race’s long-standing quest to better understand how the universe works. It’s also very expensive. And its future is unclear.

  The search for the Higgs boson isn’t just a story of subatomic particles and esoteric ideas. It’s also a tale of money, politics, and jealousy. A project that involves so many people, unprecedented international cooperation, and more than a few technological breakthroughs doesn’t happen without a certain amount of conniving, dealing, and occasional skullduggery.

  The LHC isn’t the first giant particle accelerator that aimed to find the Higgs. There was the Tevatron at Fermi National Accelerator Laboratory (Fermilab), just outside Chicago, which turned on in 1983 and finally turned off in September 2011, after a productive lifespan that included the discovery of the top quark—but no Higgs. There was the Large Electron-Positron collider (LE
P), which ran from 1989 to 2000 in the same underground tunnel where the LHC now sits. Rather than colliding relatively massive protons, which tend to create messy splashes of particles when they meet, LEP collided electrons and their antimatter siblings, positrons. That configuration made it possible to do very precise measurements—but none of those measurements revealed the Higgs.

  And then there was the Superconducting Super Collider, or SSC, to which Hewett wistfully referred. The SSC was the American version of the LHC—only bigger, better, and scheduled to be ready first. Proposed in the 1980s, the SSC planned to run at energies almost three times as high as the LHC will someday reach (five times as high as it’s achieving right now). But the LHC can boast one enormous advantage over the SSC: It got built.

  After only a couple of years of running, the LHC has bequeathed to us a genuine discovery, a particle that looks very much like the Higgs boson. It’s the end of one era but also the beginning of another. The Higgs is not merely one more particle—it’s a special kind of particle, one that can very naturally interact with other kinds of particles we haven’t yet detected. We know the Standard Model is not the final answer; the dark matter mapped out by astronomers is clear evidence of that. The Higgs could be the portal that connects our world with another one lurking just out of our reach. Having found a new particle, we have decades of work ahead of us learning about its properties and where else it might lead.