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Particle Physicists Report Possible Hints of Long-Sought Higgs Boson
22 July 2011 12:51 pm
GRENOBLE, FRANCE—Physicists working with the world's largest atom smasher may have spotted evidence of the long-sought Higgs boson. At least that's the unofficial result that has the 800 physicists here for the biannual Europhysics Conference on High-Energy Physics abuzz. Officially, experimenters working the Large Hadron Collider (LHC) at the European particle physics laboratory CERN, near Geneva, Switzerland, have merely ruled out vast ranges of potential masses for the Higgs, the particle key to physicists' explanation of how all other particles get their mass. But it's a slight excess in another region of mass that has people talking, especially as the LHC should be able to confirm or quash the putative signal within a year.
"It's exciting," says Robert Roser, a physicist at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. "I don't know what to make of it." The speed with which researchers achieved the finding shows also that the LHC is blowing away its American rival, the 25-year-old Tevatron collider at Fermilab.
Hypothesized in the 1960s, the Higgs boson solves the tricky problem of giving particles inertia and mass. Just assigning masses to fundamental particles makes particle theory produce nonsensical infinities. So somehow interactions among the particles themselves must give rise to inertia. According to physicists' standard model, a "Higgs field," a bit like an electric field, pervades empty space and drags on other particles to produce the effects of mass. And just as an electric field consists of quantum particles called photons, the Higgs field consists of Higgs bosons lurking "virtually" in the vacuum. Blast enough energy into a particle collision, and experimenters ought to be able to pop a Higgs out of the vacuum and into bona fide, if fleeting, existence.
That's exactly what physicists are trying to do with the LHC. The 27-kilometer-long ring-shaped accelerator circles 100 meters below the French-Swiss boarder and smashes protons into protons at an energy 3.5 times that of the Tevatron. The particles collide in the hearts of four huge detectors spaced around the ring, and the two largest of these detectors, ATLAS and CMS, are hunting the Higgs and other massive new particles. Kyle Cranmer, a physicist at New York University and one of 3000 researchers working the ATLAS detector, reported that the ATLAS team had shown that the Higgs does not exist in the mass range from 155 to 190 giga-electron volts (GeV), roughly 165 to 203 times the mass of a proton. It also doesn't exist in the mass range 295 to 450 GeV, he said at the meeting. Andrey Korytov, a physicist at the University Florida, Gainesville, and one of 3600 researchers working with the CMS collaboration, reported similar results: CMS rules out the Higgs in the mass ranges 149 to 206 GeV and 300 to 440 GeV.
More tantalizing, both collaborations see slightly more particles that look like Higgs than they would expect if it didn't exist. ATLAS sees the excess particles in the mass range of roughly 120 to 145 GeV. CMS sees a similar excess in the mass range of 120 to 180 GeV. Neither excess is nearly big enough to support a claim of discovery. For example, CMS's result is a "2 sigma" signal, which means the chances that random backgrounds could produce a similar blip is 2.2%, and ATLAS's is slightly stronger. "My whole life is a series of 2 sigma fluctuations," quips Vivek Sharma, a physicist at the University of California, San Diego, who is co-convener of the CMS Higgs working group. However, he says, the marginal result seems a lot more persuasive because CMS and ATLAS see excesses in the same mass range. And LHC should produce five times more data by year's end, so both experiments should be able to confirm or rule out the excess within months. "I'm hoping to have a Merry Christmas," Sharma says.
If nothing else, the results show just how fast the LHC is overtaking the Tevatron, albeit after years of delays and one major breakdown. Last year, researchers with the two detectors fed by the Tevatron—CDF and D0—had ruled out the mass range from 158 to 175 GeV. This year, they broadened that to from 156 to 177 GeV. Based on their previous results, physicists had argued for permission to keep running the Tevatron through 2014, in hopes of beating their LHC counterparts to the Higgs. But the U.S. Department of Energy, which owns Fermilab, turned down that request in January, in part on the grounds that the LHC was finally running well. Now, the LHC is zooming past the Tevatron. "I don't see this as a sad moment," Roser says. "The Tevatron ought to be outclassed by the LHC. If it's not, we [physicists] built the wrong machine."
The Tevatron will now shut down in September. Meanwhile, scientists with the ATLAS and CMS experiments say that within a year they should have enough data to probe entire viable mass range from the lower limit of 114 GeV set by previous accelerators to several hundred GeV. So, soon they'll either nail the Higgs or show that it doesn't exist. It's suddenly put up or shut up for the Higgs boson.