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

Fermilab Provides More Constraints on the Elusive Higgs Boson

The hunt for the long-sought-after particle continues in the U.S. as the Large Hadron Collider in Europe lies dormant



FERMILAB

The Higgs particle, the last piece of the Standard Model of particle physics menagerie that has yet to be observed, is running out of places to hide—if, that is, it exists at all. Fermi National Accelerator Laboratory in Batavia, Ill., today narrowed the range of mass where the Higgs might be found.

The Higgs boson, named for British physicist Peter Higgs, is believed to give other elementary particles, such as the heavy W and Z bosons, their mass, so finding it or proving it does not exist would have major implications in ground-up interpretations of how the world works.

"This is a very interesting time in particle physics, because we have this Standard Model, which explains everything we've observed and everything we know about for the last 30 years with no significant deviations. And, yet, we know that the Standard Model can't be the whole story of nature," says John S. Conway, a physicist at the University of California, Davis, and a member of the Collider Detector at Fermilab (CDF) collaboration, one of two teams involved in the new mass-range results. Many of the lingering questions in physics could be answered or at least clarified when the model's missing piece is located. "Whatever we discover," Conway adds, "it's going to be astounding."

Previous collider experiments had placed a lower bound of 114 giga-electron volts (GeV), a measure that can be used for particle mass, on the Higgs, and theoretical calculations require it to be less than 185 GeV. The new Fermilab results, from its Tevatron collider, rule out a Higgs mass between 160 and 170 GeV. (All of these constraints are at the 95 percent confidence level, according to Fermilab.)

Collider experiments such as those at the Tevatron smash particles together at extremely high energies and observe what is produced, including some exotic but short-lived particles. "We look for the signature of things we know are there and things we think might be there, like the Higgs," says physicist Craig Blocker of Brandeis University in Waltham, Mass., also a member of the CDF team. "If the Higgs had a mass in this fairly narrow range" of 160 to 170 GeV, he says, "we should have seen it, we had a good chance to see it."

Conway says the extension of the excluded Higgs masses at Fermilab is "a really exciting development." All the same, he thinks the Higgs, if it is to be found, will be first seen at the more powerful Large Hadron Collider (LHC) near Geneva, Switzerland, which is slated to come back online later this year after an aborted start-up last September. (Both Conway and Blocker are also working on physics projects at the LHC.) "It is a bit of a race" to find the Higgs, Conway says, "but if I had to bet money, I would bet on the LHC."

There are some scenarios, however, in which Fermilab—enjoying its continuing status as particle physics top dog while the LHC is sidelined—might win that race. If the Higgs happened to have a mass around 150 GeV, which Conway believes is unlikely—evidence points to a lighter particle in the neighborhood of 120 GeV, he says—the Tevatron could find it relatively soon. Alternately, with more time to gather data, the Tevatron could close in even tighter on the Higgs by inching its lower mass bound upward.

But what if the entire mass window were exhausted—if experiments showed that the Higgs, or something like it, didn't exist at all? "That would basically mean we have a very deep and fundamental lack of understanding of what is going on in the Standard Model," Blocker says. "If there's not something like the Higgs or something similar giving masses to the W and Z, we have no clue as to how that's happening."

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