Buried beneath the plains of Illinois is a monster of a machine designed to mince matter into its most fundamental parts. It’s called a particle accelerator, and it relies on 1,000 giant superconducting magnets, 700 scientists and engineers, and more than $10 million in annual electricity bills to keep on running 24 hours a day, 7 days a week.
This accelerator has a twin, tucked underneath the Alps just outside of Geneva. And researchers at each of these immense devices share one single-minded, unlikely goal: They are searching for an elusive speck of matter. The object of their search is unimaginably tiny, and so obscure that almost no one outside the world of physics has ever heard of it. Nevertheless, the stakes are high: Whichever group finds it first is likely to earn a Nobel prize.
At the center of all this scrutiny is an elementary particle known as the Higgs boson. Nobel prize-winning physicist Leon Lederman has dubbed the Higgs the “God particle,” and the divine analogy is not altogether unwarranted. If the Higgs really exists (there is still some doubt), and if it can be identified, scientists will have answered a question that was once asked only by philosophers and lunatics: Why do objects have weight? Moreover, a sighting of the Higgs could shake the entire foundation of physics-bringing with it intimations of particles that no one has yet imagined, and forces that defy all known laws. Many investigators believe a “new physics” is around the corner, and they expect the Higgs to be their guide.
At first glance, the Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Illinois, does not appear to be a place where cosmic events are poised to transpire. As you enter the sprawling 6,800-acre campus less than an hour west of Chicago, the first thing you notice is emptiness: The area is strangely barren, save for a few clusters of industrial buildings and a roaming herd of buffalo. The only hint of the power that resides here is a giant ring of dirt 4 miles around, rising 20 feet above the plains.
That loop is the outline of a circular underground tunnel, the particle accelerator known as the Tevatron. The Tevatron is so big that it’s not a machine in the conventional sense of the word; it’s a part of the landscape. Its purpose is to get a beam of protons and a beam of antiprotons moving in opposite directions as fast as possible-and in the high-powered Tevatron, fast means 99.99999 percent of the speed of light. The protons and antiprotons circulate inside a tube only a few inches wide; the rest of the tunnel is filled with wires, walkways, and 15 miles of pipes carrying liquid helium to the superconducting magnets.
Once the proton and antiproton beams are speeding along, they’re routed into a violent head-on collision. When protons smash into antiprotons, they annihilate each other in a blast of pure energy. The shards that result often include new particles-one of which just might be the Higgs.
Two detectors, each the size of a three-story house, punctuate the Tevatron’s vast ring. The proton-antiproton collisions are timed to occur within the detectors, which contain a network of wires, silicon detectors, and microchips-5,000 tons of electronics in all. The role of the two detectors is straightforward: Take a snapshot of each collision and the spray of particles that results. Sounds fairly simple, until you realize that 2.5 million collisions occur in the Tevatron’s detectors every single second. And only one collision in billions has a chance of producing the sought-after Higgs.
“These machines are microscopes, plain and simple,” explains Chris Hill, a theoretical physicist at Fermilab. We’re sitting in his office overlooking the spot where the rolling plains of Illinois meet the newly built subdivisions spreading westward from Chicago. Hill’s modest office is a jumble of computer printouts, old mathematics textbooks, and bits of chalk-an ironically untidy setup considering that he and his colleagues are determined to uncover whatever hidden order there is in the universe.
Physicists have come a long way toward ordering the universe during the past hundred years or so. First they determined that atoms are the building blocks of everything around us; then they began splicing up atoms into ever-smaller bits and pieces. The electron came first, popping out of J.J. Thomson’s British laboratory at the turn of the last century; the proton and neutron followed a few decades later. Those three particles join together in various combinations to form all 118 elements of the periodic table.
But a closer look uncovered a much more intricate structure than the proton, neutron, and electron could create on their own. In 1929, Ernest O. Lawrence built the first circular particle accelerator, which measured a mere 5 inches around. Delighted by his intriguing new tool, Lawrence began using it to explore the vast uncharted territory of the subatomic scale. Scientists quickly discovered new particles. Some, such as the muon, were related to the electron; others, like quarks, were later found to be the constituent bits of protons and neutrons. These new particles triggered a worldwide feeding frenzy, and physicists began constructing ever-larger accelerators to probe progressively deeper into the subatomic world. Fermilab and CERN (the European Organization for Nuclear Research, located near Geneva) emerged as the two heavyweights on the scene and have been alternating discoveries ever since (see “Subatomic Scorecard”).
Eventually, physicists constructed a theory known as the Standard Model, which is still in place today. The Standard Model lists all the fundamental fragments of the universe and describes the laws that govern their interactions. The culmination of 100 years of effort by thousands of the world’s top minds, the Standard Model reduces the fundamental building blocks of the universe to a set of rules basic enough to fit on a T-shirt. That’s just the kind of order and simplicity that physicists love. “What it boils down to, this chaotic world, is very few things,” proclaims Harry Weerts, an experimental physicist at Fermilab.
But there’s a catch: The Standard Model predicts that there should be a Higgs boson-without it, everything in the universe would be weightless. There would be no stars, or planets, or people, because everything would be flying through the universe at the speed of light. “All things out there get their mass from the Higgs,” explains Hill. According to Peter Jenni, a spokesman for a CERN experiment, physicists need the Higgs to explain what to a regular person might seem obvious enough: Things have mass.
But finding the Higgs boson has been problematic. Unlike many of its elementary cousins, the Higgs doesn’t normally appear as a distinct particle. Instead, it usually takes the form of an ethereal field, an invisible vapor that pervades all space.
There’s a Higgs field right now between your nose and this page, just as there’s one occupying the farthest reaches of the universe.
The Higgs field exerts more drag on some particles than on others. The more affected a particle is by the Higgs, the more weight, or mass, it possesses. Hill likens the Higgs field to the ocean, whose currents make movement tough for some creatures but not for others. “For example, you have a jellyfish that moves kind of slowly,” Hill explains. “That’s like the analogue of the top quark,” the most massive particle known. The electron, by contrast, a very light particle, “is like a little fish that can dart around.” David Miller, a physicist at University College London, has another metaphor for the Higgs field: celebrity. Imagine a party at which Madonna is a guest. Due to the hordes of clamoring fans, she finds it difficult to move around freely. Madonna is the equivalent of a heavy particle, preferentially slowed by the Higgs field. Meanwhile, a regular partygoer is like a light particle-with no papa-razzi to drag her down, she can breeze straight to the buffet.
To find the Higgs boson, scientists must first tease it out of the Higgs field. Just as a cloud forms raindrops only when conditions are right, a Higgs boson condenses out of the Higgs field only when enough energy is present. That’s why particle accelerators boost protons and antiprotons to such unthinkable speeds: to concentrate as much energy as possible in a small space.
Whenever a proton and an antiproton collide within a particle accelerator-and remember, such events occur at the staggering rate of 2.5 million per second-those explosions are picked up by the Tevatron’s two detectors, which are known as CDF and D (pronounced dee-zero). The electronic sensors within the detectors sift through the elementary rubble, ultimately discarding many of the collision events and selecting only the choicest ones for later analysis.
With a knack for understatement that only a particle physicist could manage, Franco Bedeschi, a leader of the CDF team, describes his project simply as “large.” The reality is that the work of detecting, identifying, and tracing the scores of new particles produced by every collision is something akin to tracking the pulp that’s scattered when a truck carrying ripe tomatoes jackknifes at 50 miles an hour. It’s an intricate, and messy, business. Complex computer programs compare and contrast trillions of collisions over a period of years, searching for anomalous trends in the data.
What will scientists see, if and when they manage to capture the Higgs? Like everything in this business, it’s not straightforward. The collisions themselves can be “seen” on a computer screen-each one is translated into an image that looks like a tangle of lines shooting off from a central point. But the Higgs won’t show up on any of these collision snapshots, because almost as soon as it appears, it evaporates into a pair of exotic particles called bottom quarks. Unfortunately for the Higgs hunters, bottom quarks can also be produced in a variety of other ways. Fortunately, however, the bottom quarks that are produced by the Higgs possess a specific energy level. Thus the eureka moment will come when the physicists’ computer programs have sifted through the debris of some 500 trillion collisions and found a surge of bottom quarks with the recognizable Higgs energy signature.
Higgs researchers are up against more than just the inscrutable laws of physics: they’re also battling each other. Late last year, scientists at CERN found tantalizing hints that the Higgs had already been produced in their particle accelerator. More data were needed to confirm, but CERN was on the verge of shutting down for a much-anticipated upgrade. It was a dilemma: Staying open meant delaying construction on CERN’s planned new accelerator, the Large Hadron Collider. Shutting down meant leaving the field open for Fermilab. CERN’s director opted to close.
CERN’s Large Hadron Collider, which won’t go online until at least 2006, will be seven times as powerful as Fermilab’s Tevatron and will produce 100 times more collisions every second. “It is not a small, incremental step, it’s really a new giant step,” says Peter Jenni of CERN. Fermilab’s Harry Weerts is more forthcoming about the power of the machine-to-be, conceding: “If the LHC was on right now, it would not be worth turning on the power at CDF and D.”
But whether Fermilab catches the Higgs before 2006, or CERN grabs it later, the real question is, What’s the payoff? The Higgs is expected to do a lot more than explain mass; physicists hope it will point toward a whole new foundation for 21st century physics. Some anticipated directions would be fairly straightforward extensions of the Standard Model. Others are downright bizarre, postulating the existence of various dimensions in addition to our standard three, along with a bevy of new particles. And there’s always the possibility that no one has any idea what physics will look like in 20 years.
But that doesn’t stop scientists from wondering. And for Hill and many others, the tantalizing Higgs boson holds many of the answers. What the Higgs will tell us, he says, is simple: “What are the laws of physics from here on out?”
How Physicists Search for the Higgs
1. Protons are stripped from hydrogen atoms, then pick up speed as they are fed into the
main injector.
2. Some protons are diverted into the target hall, where they are shot into a barrel of nickel metal.
A tiny percentage of the resulting debris consists of antiprotons, which are filtered out.
3. The antiprotons are sent into the main injector, circulating in the direction opposite to that of the protons. Both pick up speed.
4. Protons and antiprotons enter the Tevatron.
5. Superconducting magnets in the Tevatron accelerate the protons and antiprotons to full speed, then focus them toward the collision point inside one of the detectors.
6. The detectors take a snapshot of the new particles that are created by the collision (see inset). That data is fed to banks of computers, which sift through trillions of collisions to find the Higgs.
