Inside the subatomic race
Posted: Monday, September 24, 2007 8:12 AM by Alan Boyle
Fermilab |
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An aerial photo of Fermilab shows the futuristic
Wilson Hall alongside the cooling canal for the
Tevatron, with a prairie habitat inside the ring.
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On the surface, the Fermi National Accelerator Laboratory looks like a patch of Illinois left behind from the 1970s - or sometimes even the 1870s.
Much of Fermilab's 6,800-acre preserve, nestled amid Chicago's suburbs, has reverted to wilderness. A herd of bison roams the prairie. Decades-old frame houses and industrial buildings dot the developed areas. The tallest building is Wilson Hall, a 16-story headquarters that looks like a setting for the '70s sci-fi flick "Logan's Run."
And then there's the Tevatron.
From the surface, all you see of the Tevatron is a perfectly circular ridge and canal, running around a mile-wide island of prairie. But 30 feet down, bunches of protons and antiprotons race through supercooled high-tech plumbing. Every time those bunches collide, the chances go up ever so slightly that something remarkable will be discovered - something that nations around the world are spending billions of dollars to find.
The most exciting action at Fermilab is just beneath the surface - literally, and figuratively as well.
For now, the underground Tevatron is the world champion of particle accelerators, capable of smashing beams together at energies of up to 1.8 trillion electron volts, or TeV (the acronym that gave the Tevatron its name). But like boxer Muhammad Ali, the old champion eventually has to give way to a new generation. For physicists, the Large Hadron Collider is the up-and-comer, with expected top energies of 14 TeV.
At those energies, physicists expect to see the evidence for the only subatomic particle that's been predicted by widely accepted theory but never observed: the Higgs boson, a particle that is thought to be behind the fundamental quality of mass. The top reason for building the $8 billion Large Hadron Collider on the French-Swiss border is to bag the Higgs boson. After months of delay, the LHC is due to start up next spring.
The old champion may have one more surprise punch left, however: The word is that something interesting has shown up in data from the Tevatron's DZero detector - interesting enough to extend operations into 2009 and maybe into 2010.
Could the aging Tevatron actually scoop the LHC and find the Higgs boson first?
"It's a good story now for physics," Pier Oddone, Fermilab's director, told me last week. He said the "David and Goliath type of situation" pitting the Tevatron against the LHC is just part of the story.
"It's very seldom that you're opening, well, I call it a continent - it's opening some great expanse of new territory for the first time, where you truly don't have a clue as to what's going to turn out," the Peru-born physicist said.
Oddone isn't fazed by the fact that merely detecting the Higgs would stretch the Tevatron's capabilities to the limit, and that it would be up to the LHC to explore the rest of that subatomic continent.
"Columbus only got to the beach. There was a whole continent out there. But he got a lot of mileage out of that," he said with a smile.
DZero: Inside the cathedral
If Oddone's metaphor holds water, the DZero detector serves as one of the vehicles in Fermilab's fleet to find that new continent.
To get to the detector, you enter one of the industrial buildings and take an elevator four stories down. The Tevatron does its proton-smashing underground so that there's minimal electromagnetic interference from topside to spoil the results. When the ring is at work, DZero and Fermilab's other detector, the CDF, monitor the violent sprays of radiation given off every 132 nanoseconds by particle collisions. Computers sift through the resulting data, looking for telltale patterns in the subatomic blast.
Alan Boyle / msnbc.com |
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DZero co-spokesperson Darien Wood stands in the
detector's "cathedral," with muon detectors on the
left and the calorimeter assembly on the right.
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You don't want to be in there when the beam is on. Radiation warnings are posted all over the control room and the stairwell leading to the detector room itself. The Tevatron was shut down for summer maintenance - which is why DZero co-spokesperson Darien Wood, a physics professor at Northeastern University, was able to take me on last week's tour. Just to be safe, Wood wore a dosimeter around his neck to monitor our radiation exposure levels. (We came out clean.)
It's hard to describe what it's like to stand inside a detector unless you've scrambled around through the crawlspace of an underground construction site - or perhaps the hold of an ship. You have to scramble through iron structures and climb aluminum ladders to get into the guts of the 45-foot-high machine, which are mounted on rails so that they can be pushed apart for maintenance.
Wood took me into DZero's "cathedral" - a high-ceilinged alleyway with a wall of muon detectors on one side and the detector's tanklike calorimeter assembly on the other side. The muon detectors look like metal shingles, slightly overlapping so that particles thrown off by the proton collisions can't sneak through. Wood told me that the designers studied fish scales to figure out how to design the system.
The calorimeters measure the energy given off by the collisions. Inside the calorimeters are sensitive particle-tracking devices, hidden like the meat inside a corn dog. And running through the very center, like the stick in that corn dog, is the beam pipe. That's the pipe where the speeding bunches of protons and antiprotons are focused into each other for the collisions ... where the Higgs boson might fly free for less than a nanosecond before shattering into other particles.
"Many of these particles - the Higgs or the top quark - never make it out of the beam pipe," Kurt Riesselmann, deputy head of Fermilab's public affairs office, told me.
Last week, the calorimeter segments were pulled apart just enough that Wood could shine a flashlight into a crawlspace. The 2-inch-wide beam pipe ran across the space, about 10 feet away. Garish purple ribbons were tied around the pipe with their ends hanging down.
Wood said the ribbons serve as a reminder for the hardhat maintenance workers. "The last thing we want is to have someone bump the beam pipe," he told me.
CDF: Getting a tune-up
About a mile away, the maintenance work was in full swing at the Tevatron's other big particle detector, the Collider Detector at Fermilab, or CDF.
When the detectors were first built in the 1980s, the CDF was optimized for measuring particle momentum, with a big magnet and a better tracker. DZero, meanwhile, was optimized for measuring energy, with a finely segmented calorimeter. Both detectors were upgraded in the 1990s for the current campaign, known as "Run 2," and now they have roughly similar capabilities, said University of Florida physicist Jacobo Konigsberg, co-spokesperson for the CDF experiment.
Alan Boyle / msnbc.com |
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A life-size mural showing Fermilab's CDF particle
detector hangs down from scaffolding in an
underground assembly hall. The real thing sits
right next door, hooked up to the Tevatron.
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"At the end, both experiments are doing very good physics," Konigsberg told me.
The last time the CDF was out in the open was during the preparations for Run 2, when the 30-foot-high cube of electronics was rolled out to the underground assembly hall for its upgrades. Today, a life-size mural of the CDF's front face hangs from the scaffolding in the hall. You can see what the real thing looked like back then in this archived picture, and even watch a 2001 time-lapse movie showing the CDF being rolled back into place (GIF or QuickTime).
During this month's shutdown, workers were fixing the CDF's cryogenic cooling system, which meant the delicate equipment at the detector's center had to be opened up.
We looked over the shoulders of the hardhat workers as they made the repairs - and tried to stay out of the way of the technicians who carried metal scaffolding as they marched down the hallway. The tolerances on the equipment are so tight that serious damage could be done if the replacement washers aren't just the right size.
"It's all experimental science - it's unique," Konigsberg said. "We build it as we go, we learn it as we go. ... That's why it's called an experiment."
The scientists and engineers working on the Large Hadron Collider at CERN will likely have to go through a similar learning curve. "They're going to have a lot of difficulty accessing their detectors," Konigsberg said.
Both sides now: Friendly rivals
During lunch at Fermilab's Wilson Hall, Konigsberg exchanged observations with Dmitri Denisov - who, like Wood, serves as a co-spokesperson for the DZero experiment. Denisov came to the United States from Russia in the 1990s, hoping to work on the Superconducting Super Collider in Texas. After Congress pulled the plug on the supercollider in 1993, Denisov made his way from the Texas plains to Fermilab's prairie.
"It's very exciting," Denisov told me. "Look, my workday starts before 8 in the morning, and finishes well past 8 in the evening."
But Denisov is stingy with information about the search for the Higgs, even in his conversations with Konigsberg. Although DZero and CDF are both part of Fermilab, the two teams are friendly rivals in the quest for scientific breakthroughs. A couple of months ago, for instance, the DZero team announced the discovery of the "triple scoop" baryon just three days before the CDF team made their own independent announcement.
"People compete until they can't, and then they join forces," Konigsberg said.
That may be the case with the Higgs boson. Konigsberg noted that the properties of two other subatomic particles - the top quark and the W boson - suggest that the Higgs may be lighter than physicists thought, and just might be sitting in the sweet spot for the Tevatron.
"This issue has been moving in a way that has put it in our reach," Konigsberg said.
In the end, the discovery of the Higgs may come in small steps, with one research group reporting tentative observations that are later confirmed by another. "You'll publish if you can, and we'll publish if we can, and then we'll combine," Konigsberg told Denisov at lunch.
Even if neither team at Fermilab comes up with the definitive answer, their work can serve as a guide - and a goad - to their friendly rivals at the Large Hadron Collider in Europe.
"I think we're helping them in an indirect way," Denisov said.
More dispatches from the Big Science Tour:
