This article is from the In-Depth Report Farewell to the Tevatron

Opposite Spins: The LHC Accelerates Higgs Search as the U.S. Shutters Its Tevatron

Europe's Large Hadron Collider is extending its unprecedented experimental run as the U.S. prepares for a disappointing shutdown of its marquee collider
LHC from above

CERN/Maximilien Brice

There can be only one top-dog collider in physics, one ring-shaped machine to rule them all. Since late 2009, when the Large Hadron Collider, or LHC, started up outside Geneva and quickly became the most powerful collider in history, particle physicists' eyes have been wandering steadily toward Europe and away from the U.S., where the Tevatron in Illinois had long held sway as the world's best.

The shift in power was neatly expressed in a pair of recent and apparently unrelated announcements from the laboratories that operate the two accelerators. First came the January 10 news from Fermi National Accelerator Laboratory (Fermilab), where the 6.3-kilometer particle racetrack of the Tevatron is housed, that the U.S. Department of Energy (DoE) had decided against a proposed three-year extension of the collider. The storied Tevatron, which has racked up numerous discoveries since coming online in the 1980s, will now cease operations in September, it was announced. Three weeks later came an almost mirror-image statement from CERN (European Organization for Nuclear Research), the laboratory for particle physics that operates the LHC, which occupies a ring 27 kilometers in circumference. Rather than shutting down the machine for upgrades and repairs in 2012 as had been scheduled, CERN now plans to operate the LHC through 2011 and 2012, mostly uninterrupted, in the hopes of finding new physics. The laboratory will then prepare the LHC for a period of higher-energy operation starting in 2014.

Both machines accelerate beams of elementary particles to nearly light speed, steering them around massive rings with superconducting magnets before smashing together opposing beams in head-on collisions. (As with automotive racetracks, larger collider rings have gentler curves that allow for more energetic motion.) The idea is that by sifting through the debris from these collisions, physicists can identify short-lived, exotic particles and probe the high-energy conditions that existed in the crucial first instants after the big bang.

The call to extend the LHC's run was not influenced by the decision to shutter the Tevatron, according to CERN spokesperson James Gillies. "The decision to run through 2012 was motivated by the good performance of the LHC last year pointing to the possibility of good physics by staying at this energy for two years of running," he says.

One distinct possibility is that the LHC's two major detectors might uncover—or disprove the existence of—the Higgs boson, a particle that is hypothesized to lend other particles their otherwise unexplained mass. "Combining the data from the two big, general-purpose experiments, ATLAS and CMS, you never know," says Duke University physicist Alfred Goshaw, who chairs the U.S. institutional board for the ATLAS collaboration. "There would be a very good possibility to observe or exclude the Higgs boson" across a wide range of possible masses for the particle, he says.

The high-profile Higgs search was the main draw for keeping the Tevatron running, says Fermilab physicist Dmitri Denisov, co-spokesperson for DZero, one of the two major Tevatron experiments. "With this extended three years of running and a low-mass Higgs boson where it's supposed to be, we would be able to see 3-sigma evidence for it," Denisov says. (A 3-sigma result refers to a greatly reduced likelihood of a statistical fluke.) Goshaw calls it a "scientific disappointment" that the physics community will not have both colliders in the hunt.

After all, if the Higgs proves to be near the lower end of its range of possible masses, as experiments indicate is likely, the Tevatron would have had a good shot at finding it—and maybe even beating the LHC to the punch. "It turns out the most difficult mass range for the LHC is the very low-mass Higgs," Goshaw says. At the energies reached in LHC collisions, the dominant decay products of a low-mass Higgs, namely a beauty quark and its antiparticle, are buried among beauty quarks and beauty antiquarks produced by other processes. So the LHC must turn to a less common and hence more data-intensive decay of the Higgs into photons.

The DoE cited a challenging fiscal climate in declining to keep the Tevatron running. "It was definitely a disappointment, as you can expect," Denisov says. "We really made in our view a very strong proposal—and not only in our view," he adds, noting that the extension request had received broad support from review committees and from the physics community at large. "But I don't think we should overplay negatives here. The Tevatron program has been extremely successful." And analysis of data gathered by the experiments there will continue for years after the collider itself is switched off, potentially leaving the door open for the outgoing giant of the physics world to deliver one last big score. "Even with data we will get by the end of this year, we will be able to exclude the Higgs if it doesn't exist," Denisov says.

Ruling out the Higgs boson's existence would throw a quantum wrench into the smoothly churning gears of the standard model of particle physics, a description of the physical world that has survived decades of intense experimental scrutiny. Regardless of what happens in the Higgs chase, Goshaw notes that the LHC has the potential to crack open similarly new realms in our understanding of the universe. "There are so many other attractive theories that we could very easily break into—dark matter, supersymmetry, black hole production," he says. "The Higgs is sort of the highlight, but if we don't discover anything, that would be absolutely incredible."

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