To find out more about the Large Hadron Collider check out our companion web exclusive article Beyond the Higgs.
Near the west end of Lake Geneva in Switzerland, buried under the river plain of the Rhône, workers are fitting together the final pieces of the machine that hopes to unlock one of the biggest mysteries of the universe. It has taken over 20 years, $8 billion, and the combined efforts of more than 60 countries to create this extraordinary particle smasher, the Large Hadron Collider, or LHC, built and operated by CERN, the European physics consortium.
The “large” in Large Hadron Collider is something of an understatement. “Enormous” is closer: The collider’s underground tunnel carves a circle 17 miles in circumference, traversing the border between Switzerland and France. At four locations it passes through caverns crammed with detectors the size of buildings. In a deliberately constructed rivalry, two of these detectors—along with their armies of scientists, engineers, and technicians—will vie with each other to discover the obscure but wildly important particle known as the Higgs boson.
According to the most accurate scientific theory ever created—known as the standard model—all of space is filled with a mysterious stuff called the Higgs field. Unlike magnetic or gravitational fields, which vary from place to place (things weigh more here than on the surface of the moon, for instance), the Higgs field is exactly the same everywhere. What varies is how the different fundamental particles interact with it. That interaction, the theory goes, is what gives particles mass. In a nutshell, the Higgs field is what makes some particles (like protons and neutrons) relatively heavy, others (like electrons) subatomic lightweights, and still others (like photons) utterly massless. If photons weren’t so light, you’d be shredded by a photon hailstorm every time you lazed under a sunbeam. Then again, if protons and neutrons weren’t so heavy, you wouldn’t be there to sunbathe anyway: Without mass and its affinity for gravity, there’d be no galaxies, no stars, no us.
How does the Higgs work this magic? British theoretician John Ellis likens the Higgs field to a flat field of snow. Try to get across it in hiking boots and you will sink in and take forever. Snowshoes would be faster, and with skis you could glide across the field swiftly and easily. In the parlance of physics, “slow” is another way of saying “heavy.” So by analogy, your mass depends on some fundamental physics attribute, equivalent to snowshoes or skis, that affects how a particular type of particle passes through the Higgs field.
The Higgs boson is supposed to be the endower of this attribute; it is what determines if a particle can glide along effortlessly like a photon or if it must trudge like a hefty proton. The trouble is that nobody knows exactly what a Higgs boson is like or even if it really exists. It must be extremely heavy, or other lower-energy facilities, like Fermilab outside Chicago, would already have detected it. But it cannot be too heavy, or the theories that predict its existence would not work.
By design, the LHC is the first accelerator capable of exploring the full range of energies within which the Higgs boson is thought to exist. If the LHC finds the Higgs, it will verify the last, grandest aspect of the standard model and solve the ancient question of just what mass is. If the LHC fails to find the Higgs, the standard model will have to be reevaluated from the ground up. At stake is a fundamental part of our understanding of how the universe works.
Peter Limon, an American from Fermilab, hands me a hard hat and a metal box containing breathing apparatus. “You’re entering an industrial area,” he says. “Watch out for bicycles.”
We’re about to take an elevator more than 300 feet belowground, into a tunnel containing the biggest, most violently energetic particle collider the world has ever known.
The endless, gently curving tunnel is so crowded with massive high-tech equipment that there isn’t much room for any transportation other than a bike. “Best way of getting around down here,” Limon explains.
What’s filling the tunnel is the beam pipe: the hardware used to accelerate subatomic particles (protons, mostly) to 99.999999 percent of the speed of light. From the outside, the beam pipe looks like a series of huge steel barrels, connected end to end and brightly painted in reds, oranges, and blues; it stretches off into the distance like a giant oil pipeline. Many of the barrels bear a stenciled sign that betrays the international nature of the project. Some are from Italy, others from Japan or the United States. One of the barrels is cut away, and Limon shows me the complexity within. The beam pipe actually contains two beam lines, tubes just an inch and a half across, inside of which streams of particles will speed around the circuit of the LHC. Surrounding the beam lines is a forest of pipes, electronics, and ultrapowerful magnets. When the machine is switched on for the first time at the end of this year, particles will make a lap around the LHC in less than one ten-thousandth of a second.
Keeping those particles on track requires serious bending power from more than 1,200 superconducting magnets, each of which weighs several tons apiece. Each magnet must be kept at –456 degrees Fahrenheit—colder than the void between galaxies—requiring CERN to build the world’s biggest cryogenic system to handle the 185,000 gallons of liquid helium that will be used to chill the magnets.
Particles will circulate in opposite directions in each beam line—clockwise in one, counterclockwise in the other. The individual beam lines will keep the racing particle streams separated—except at four points around the ring where physicists will deliberately allow the streams to cross. At those spots, the LHC physicists will observe the resulting mayhem with detectors of staggering scale and complexity.
Standing at one of these collision points, I try to imagine the energy involved. “If I were down here when the beam was operating, would it be highly radioactive and dangerous?” I ask. “If you were down here when the beam is operating,” Limon replies, “it would be highly radioactive and fatal.” There will be 600 million particle collisions per second, and although the particles themselves are mere specks—less than a million millionth the size of a gnat—their collective energy will be that of an express train. Once set in motion, a stream of particles might circulate for 10 hours before needing to be refreshed. During that time, it would travel more than 6 billion miles, enough to get to the planet Neptune and back.
“I think this is the most complicated thing that humans have ever built,” Limon says, proudly.
The LHC’s subatomic fireballs will be the highest-energy particle collisions ever seen on Earth. This is uncharted territory: The collisions at LHC could spray out strange new kinds of matter, unfurl hidden dimensions of space, even generate tiny glowing reenactments of the birth of the universe. In short, there is more than just the search for the Higgs going on at the LHC. “We don’t even know what to expect,” says French physicist Yves Schutz. “We’re now in a domain of energy that nobody has ever explored.”
Schutz is focusing on one of the other projects here. His experiment is A Large Ion Collider Experiment, or ALICE (whimsical acronyms are a way of life here), which will smash ultraheavy lead ions together to create a miniature fireball to mimic the first split second after the Big Bang. In spite of its name, ALICE is one of the two smaller experiments on the ring. The other, LHCb, will seek to understand why the universe contains matter rather than antimatter or, worse, nothing at all (to find out more about these other experiments, see the online version of this article at www.discovermagazine.com).
But the stars of the LHC are the two rival detectors, set diametrically opposite each other on the ring. In one corner is ATLAS; squaring off a little over five miles away is the CMS. Together, these two detectors cost a cool $850 million, and although their designs are quite different, they are looking for exactly the same things.
Touring these vast experiments, one wonders why CERN decided to double its efforts and costs. Why not pour all its resources into one detector to ensure CERN’s place at the top of particle physics as quickly as possible?
The reason is a fundamental principle of science: Experimental results must always be confirmed through duplication. In earlier decades, there was more or less a parity of atom-smashing capabilities between the United States and Europe, each leapfrogging and confirming the results of the other in turn. But when America abandoned its plans to build the Superconducting Super Collider in 1993 (with $2 billion spent and 14 miles of tunnel already dug in Texas), it left the LHC without a peer. So to prevent any embarrassing excursions into the scientific wilderness, CERN decided to build two detectors with independent teams, each to check the results of the other. As the exact properties of the Higgs are unknown, two different designs also allows CERN to hedge its bets.
As I arrive above the surface of the CMS, British physicist Dave Barney explains that the name of his experiment stands for Compact Muon Solenoid. A solenoid is basically a cylindrical electromagnet that generates a very uniform magnetic field inside the cylinder; the uniform field makes it easier to calculate the momentum of particles produced from collisions. The CMS electromagnet is “compact” only in the sense that it is incredibly dense. At 40 feet long, it is the biggest superconducting solenoid ever made, costing $65 million, weighing about 485,000 pounds, and containing as much iron as the Eiffel Tower. From the outside it looks like a huge steel bullet protruding from the center of a steel cylinder some 50 feet tall, covered in cables and instruments and surrounded by scaffolding. “The magnetic field is immense; if they switched it on now and you had steel-capped shoes, you’d fly over there,” Barney says.
The magnet will deflect the spray of new particles created by the colliding streams, while other instruments around it will detect the paths of those particles, soak up and register their energies, divining what they are and where they came from. Many of the particles will survive only a trillionth or less of a second before decaying, but that will be long enough to leave a telltale trail. The vast size of the CMS is a function of the immense energies involved. The bigger the energy, the stronger the magnet needed to deflect the particles and the more space required to register their properties. “If you want to build the biggest bangs in the world,” says Barney, “you have to give them space to breathe.”
The CMS is being constructed aboveground in massive sections, each of which is then lowered by crane underground in a process that takes 10 hours. Down below, the half-assembled slices resemble a futuristic spaceship. “It’s like Star Wars,” Barney says. “You know how you’re always seeing vast machines moving about. That’s what it feels like to me.” We watch as one of the pieces rises imperceptibly up on an orange-skirted “hovercraft” and see it glide slowly and silently toward its mate.
Barney has been working on a detector in the CMS for more than 10 years, and he is fiercely proud of it. He refers to the rival ATLAS experiment, only half jokingly, as “the enemy.”
ATLAS stands for A Toroidal LHC Apparatus. “Let me show you what a real experiment looks like,” says American physicist Steve Goldfarb, on loan from the University of Michigan, at the door of the ATLAS hangar. Goldfarb explains that instead of using one dense magnet close to the center of the machine, as in the CMS, ATLAS has an array of multiple smaller magnets, with lots of empty space for particles to pass between them. The upside here is that the ATLAS team didn’t have to worry about building the biggest solenoid the world has ever seen. The downside is that the resulting magnetic field is complicated, with loops and whorls that will make calculating the particles’ trajectories a major headache. Using multiple magnets also makes the detector far too big to be built in pieces that are lowered from the surface. Instead, ATLAS has had to be constructed entirely in place.
The activity is intense. I count seven stories of scaffolding and numerous hard-hatted workers. We are on a gantry, level with the center of the machine, and as we walk along the side of the detector, all 150 feet of it, Goldfarb points out the casings of the various magnets. The central chamber is barely visible through the surreal spaghetti bundles of cables. At the far end of the chamber are the ends of eight magnetic coils, each pointing toward the center of the central chamber. It looks eerily like a vast portal to another universe.
Both ATLAS and the CMS plan to focus the energy of the LHC beams into a single pinprick of space just a fraction of an inch across. That maximizes the number of collisions and the chance that new, ultraheavy particles will emerge from the wreckage. In these collisions, energy gets transformed into mass. The more energy that goes in, the more massive the particles that can come out. Since the LHC will pack more energy than any previous accelerator, it should also create more massive particles than any ever before seen—including, Goldfarb hopes, the elusive Higgs boson.
Across the Atlantic, the Americans still hope to pull off an 11th-hour upset. For a few more months, Fermilab’s Tevatron, in Batavia, Illinois, remains the world’s most energetic accelerator. Although the Tevatron is near the end of its lifetime, it still has a chance to find the Higgs boson before the LHC can be fired up. Could the Tevatron really pip the LHC at the post? “I think it’s going to be pretty tough for them,” says Ellis, “but personally, I wish them luck. As a theorist, I’m happy to cheer all the horses in the race.”
At the LHC, Goldfarb is obsessed about the precision of the ATLAS detectors. “We need to know the position of each detector to the thickness of a human hair in a machine the size of half a football field.” He tells me that the detectors will generate a million gigabytes of data per second. “That’s several hundred thousand DVDs per second. We don’t know how to burn that many DVDs that fast or what we would do with them.”
The first step is to filter out the dull from the profound. Behind protective concrete, banks of computers are ready to do the initial sifting work. After that, the data will pass up to the computing center, where the real analysis will begin. Even there, data from the two experiments will be kept separate, with security systems in place to prevent peeping. “We can’t have the experiments eavesdropping on each other,” says computer communications head François Grey. “We want completely independent observations.” The computing effort is a major challenge in its own right and is one of the oft-cited justifications for a project like this. The last time that scientists at CERN got together and tried to solve a vast computing problem posed by their particle physics experiments, they came up with the World Wide Web.
“What you see is a huge effort, but what you get out is enormous,” says Goldfarb. “We’re going to understand our universe better. Now there are still too many numbers that we have to measure. We’re still hoping for some simple rule, one simple particle at the basis of all this.”
What if neither team finds the elusive particle or rule that explains everything—will all this have been worth it? “This whole complex detector probably only costs the same as one super next-generation bomber to drop bombs better,” Goldfarb says. “But the sole purpose of this is to figure out the universe. I’d rather have people working on something like this.”