ATLAS, oneof the two giant underground detectors
that will search for the Higgs boson.
Image courtesy of © CERN
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.”
To find out more about the Large Hadron Collidor check out our companion web exclusive article HED: Beyond the Higgs
