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The rise of the collider

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The Tevatron, a circular particle accelerator at the Fermi National Accelerator Laboratory (also known as Fermilab) in Illinois, was the second-highest energy particle collider in the world after the Large Hadron Collider (LHC). (Photo: Reidar Hahn/Fermilab)

The general idea is simple. Magnets accelerate protons around a 17-mile ring to nearly the speed of light. These particles collide with other protons traveling the opposite direction. From the wreckage of these subatomic collisions, giant detectors search for clues to new directions in physics. This is how scientists recently discovered a new particle, possibly the Higgs boson, that could be proof of a theoretical force called the Higgs field, a field that may exist throughout the universe and be responsible for giving elementary particles their mass.

Every aspect of this event, which took place at CERN's Large Hadron Collider (LHC) in Switzerland over summer 2012, is the accumulation of decades of hard work from countless scientists and graduate students.

Before anything like the LHC is even built, a laboratory like CERN must establish a path of discovery, determining which particles to search for or what physics to better understand. Theorists help chart this course by predicting what lies beyond the known observations. The laboratory secures funding, while scientists engineer and construct the accelerator and all its millions of complex parts.

Creating a $10 billion collider is never simple. For the previous record-holding supercollider, the Tevatron, radical ideas were tested; while some failed, others revolutionized the field.

Today, three scientists — all AAAS fellows and retirees of Fermi National Accelerator Laboratory in Illinois — look back on their contributions to the Tevatron and how their work led to the new particle observed in 2012 at the European laboratory CERN.

The missing pieces

For Bill Bardeen, physics rarely came up at the family dinner table — this, despite the fact that he studied the tiniest elements in nature as a theoretical particle physicist. His brother, a cosmologist and AAAS fellow, examined the universe on the largest of scales. His sister married a physicist, and his father, John Bardeen, is the only person to ever win two Nobel Prizes in physics.

Bardeen says he chose elementary particles less for the unique career path and more for the excitement of hitting a field in full bloom. It was the 1960s, and new theories were setting a course of discovery for decades to come.

His decision to enter theoretical physics led him to a refining of a then-novel theory, which is known today as the Standard Model. This series of calculations involving all the hundreds of known particles and their four fundamental interactions (the electromagnetic, weak, strong, and gravitational interactions) has evolved into the most widely accepted theory to explain particle physics. It has guided experiments at the Tevatron and the LHC in the search for a Higgs particle.

Bardeen realized in 1969 that for electroweak theory to truly unify two of the four fundamental forces of nature — the electromagnetic and weak force — theoretical elementary particles called quarks must play a significant role. Quarks restore symmetry by canceling anomalies and other strange phenomena plaguing quantum field theories, like the early Standard Model. With this method of calculating electroweak theory, it was revealed that a hidden field pervading the universe — such as the Higgs — may be the mechanism that gives these weakly interacting particles mass.

"The idea that you could have a calculable theory of electroweak interactions was sort of the revolution that led a lot of people to take that kind of theory more seriously," says Bardeen.

The new theory correctly predicted the discovery of the top quark at the Tevatron, along with several other particles — a track record that made the existence of the Higgs field increasingly likely. As head of Fermilab's theoretical physics department, Bardeen continued to develop award-winning theories and systematic calculations — fuel for supercolliders in the hunt for new particles.

The magnet man

"When they built this magnet it didn't work at all. It was a disaster," says retired physicist Alvin Tollestrup about the early work on the Tevatron collider at Fermilab. "When they understood what the trouble was, that was about the time I came."

Tollestrup had taken a six-month sabbatical from teaching at the California Institute of Technology when he arrived at Fermilab in 1975. He stayed for 37 years. The reason: colliding beams.

Rather than firing particles at a fixed target as in previous accelerator experiments, Fermilab's founding director Robert Wilson pushed for a design that could collide two streams of protons and antiprotons to double the power and increase the energy of the collisions. This, he believed, would release high-energy particles that had never been seen before, particles like the Higgs boson.

Yet the magnets they needed for the experiment lacked the accuracy that was required of them. "If you wanted superconducting magnets in '75 you would have to go to one of the little companies that made these things," says Tollestrup. "They would order the niobium and titanium and process it and if you're lucky a year later you might get a magnet."

For half a day every day, Wilson visited Tollestrup and his colleagues in the workshop, where they sifted through designs for the Tevatron magnets. The team settled on an idea to use the longest possible magnet that could fit in a circle, which would allow the low magnetic field needed. Yet one scientist, Helen Edwards, found vulnerabilities in this system and suggested the opposite: Shorten the magnets as much as possible and add more correction coils to adjust the beam.

"That was a crucial accident that made things work," says Tollestrup. "The original magnets we were working with would never have worked."

Wilson then amassed an industrial amount of niobium and titanium — the raw material for the magnets. He built a magnet factory, where the scientists could assemble, freeze, and test up to 3.5 magnets a day, cutting the entire process for a single short magnet down to a week. As a result, the assembly line produced nearly 1,000 magnets for the Tevatron.

These brief missteps and bold achievements in constructing the Tevatron were the driving force behind the LHC's ultimate precision.

"If you take any of the magnets of any of the superconducting machines and take a cross section of them," says Tollestrup, "you can see the DNA of the Tevatron in all of them."

Like the LHC, the Tevatron had enormous detectors for analyzing the particle collisions. Once running, those detectors discovered the top quark in 1995, the last of the quarks predicted by the Standard Model.

"All this stuff was happening because people were taking chances that it would work. And it did," says Tollestrup.

The gamble

John Peoples, the third director of Fermilab, took over in 1989 as a new threat was emerging — the Superconducting Super Collider (SSC) — a machine being built in Texas that was designed to be more powerful than the Tevatron and even the LHC. The success of the Texas collider would mean the destruction of the Tevatron.

With this uncertain future, Peoples and the accelerator laboratory began to try to reach byond the Tevatron, initiating a phased shift from high-energy to high-intensity physics, pushing for more focused searches with more intense particle beams.

When the new Main Injector ring didn't boost the energy and accuracy of the Tevatron as much as was promised to the Department of Energy (DOE), Peoples and his colleagues developed a plan to install a low-energy magnet ring within the Main Injector that would deliver the promised upgrade.

"Going to DOE with a crazy proposal, even though it was cheap, that was very upsetting to them," says Peoples.

So he gambled on a demonstration. If the test magnets worked, DOE would fund the ring. If they failed, the program would stick with the original magnets.

The new magnets exceeded expectations. Once installed, the Main Injector not only increased luminosity — the number and accuracy of the particles — it maximized energy, freed up bottlenecks, and established a way to manufacture and launch exotic neutrino particles to distant underground detectors.

Neutrinos were capturing the fascination of physicists at that time. These mysterious, shape-shifting particles pass through nearly every type of material — about 60 million are zipping through the tip of a fingernail at any given moment — and they morph into other types of neutrinos as they travel.

Peoples and Fermilab readied the accelerator chain for the transition to this new frontier of physics. Though the SSC lost funding, the LHC smashed its first particles in 2009. As a result, Fermilab lost funding for the now inferior Tevatron and shut it down.

"When the SSC went down, our strategy fit just as well with the LHC," says Peoples.

Yet the Tevatron's demise and the LHC's success unveiled cracks in the U.S. funding structure, which led to the shift in high-energy physics overseas.

"The project for the main ring, the SPS [predecessor to the LHC], the discovery of the W and Z bosons, the discovery of the top quark, the Tevatron, the LHC — these things were all sort of [interwoven] in this competition and cooperation," says Peoples.

In the half-century search for a Higgs boson, the Tevatron ruled out certain energy ranges where the particle could be hiding. Then in December 2011, both the Fermilab and CERN teams announced they had found new clues to the particle, though more data was needed. This pointed the way for the LHC discovery this July of a Higgs-like particle — the last remaining part of the Standard Model and a possible key to a new world of physics.

"(CERN) built a beautiful machine, really nice," says Peoples. "But they had a lot of benefit of hindsight to pick up things that (Fermilab) had done that were radical at the time."

 

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The Tevatron, a circular particle accelerator at the Fermi National Accelerator Laboratory (also known as Fermilab) in Illinois, was the second-highest energy particle collider in the world after the Large Hadron Collider (LHC). (Photo: Reidar Hahn/Fermilab)
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