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Last Hurrah: Final Data From U.S. Collider Show Hints of Higgs Boson

7 March 2012 4:00 am
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Fermilab

The plot. The dashed line shows the limit on Higgs production physicists expected to set. The solid line reveals the observed excess.

The hunt for the Higgs boson, the most sought-after particle in physics and the key to physicists' explanation of how all particles get their mass, is heating to a boil. This week, scientists working with an atom smasher in the United States called the Tevatron, which shut down in September 2011, reported that, having analyzed all the data they'll ever get, they see hints of the Higgs. The signs are not strong enough to clinch a discovery, but they jibe nicely with hints reported last year by researchers working with Europe's higher-energy Large Hadron Collider (LHC).

The result "doesn't make me more convinced [that the Higgs is there], because I'm already convinced," says Gordon Kane, a theorist at the University of Michigan, Ann Arbor. "But I hope it makes a larger fraction of the audience out there convinced."

Located at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, the Tevatron smashed protons into antiprotons to blast into fleeting existence subatomic particles not ordinarily seen in nature. Those collisions occurred within two massive particle detectors, known as CDF and D0, which strived to identify new particles as they quickly decayed into combinations of more familiar ones. In their final data sets, both the CDF and D0 teams see more candidate Higgs decays than one would expect from random background processes, scientists reported today at the conference Rencontres de Moriond in La Thuile, Italy.

The excesses are in line with Higgs hints reported in December 2011 by researchers working with the LHC at the European particle physics laboratory, CERN, near Geneva, Switzerland. The LHC smashes protons into protons within two even bigger detectors that are hunting the Higgs, called ATLAS and CMS. The ATLAS and CMS teams both see excesses of candidate Higgses with a mass of about 125 giga-electron volts (GeV), or 133 times the mass of the proton. The CDF and D0 teams see candidates with roughly the same mass although with poorer mass resolution. "If you look at what ATLAS sees, at what CMS sees, and at what CDF and D0 see, it starts to look like a consistent picture," says Fermilab's Rob Roser, co-spokesperson for CDF.

Key differences between the Tevatron and the LHC also suggest that researchers with both machines are seeing Higgs bosons with the properties predicted by physicists' standard model. For example, a Higgs would emerge from a proton-antiproton collision at the Tevatron slightly differently from the way one would spring from a proton-proton collision at the LHC. Also, the CDF and D0 teams search for Higgses decaying into a different combination of particles from the ones sought by the ATLAS and CMS teams. Still, the results from Tevatron and the LHC roughly agree with each other and with standard model predictions.

The Tevatron signal isn't overwhelming. Physicists measure the strength of a signal in multiples of the uncertainty in it, denoted sigma. A higher multiple implies a lower chance that background decays could mimic the signal and signifies a more robust result. Together, CDF and D0 see a 2.2-sigma excess. That combined result is as strong as either the ATLAS or CMS result alone, says Fermilab's Dmitri Denisov, co-spokesperson for D0. But, he says, it falls short of the 5-sigma standard for discovery or even the 3-sigma standard for "evidence" of the Higgs.

All hints will be put to the test by the end of the year, as the LHC collects more data. And if a 125-GeV Higgs does emerge, will Tevatron researchers deserve some of the glory? "I think people would have to mention us in the same sentence" as the LHC teams, Roser says. But, Denisov notes, "whoever passes 5 sigma will make the discovery, and the Tevatron cannot do that."

For now, some urge caution. "It's exciting," says Howard Gordon of Brookhaven National Laboratory in Upton, New York. But "we should wait until the end of the year to say anything more definitive."

As for what might have been, Denisov says physicists presented the result to Fermilab Director Pier Oddone a week earlier. "At the end, we said, 'Look, Pier, we want to run the Tevatron again,' " Denisov says. In response to the joke, he says, Oddone smiled.

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