New mass limits could even the playing field for finding key elementary particle

By Ron Cowen

Web edition: March 16, 2009

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View Larger Version | A new analysis of data from two ongoing experiments at the Fermi National Accelerator Laboratory shows that the Higgs boson cannot have a mass between 160 and 170 billion electronvolts (GeV). Previous experiments limited the proposed mass of the theorized Higgs particle to between 114 and 185 GeV.

Credit: Fermilab

In the race to find the legendary Higgs boson, an old U.S. atom smasher has inched ahead of its spanking new and more powerful European rival. Discovery of the theorized subatomic particle would complete the standard model of particle physics.

Physicists announced March 13 that an analysis of two ongoing experiments at the Fermi National Accelerator Laboratory’s Tevatron particle accelerator in Batavia, Ill., has put new limits on the allowed mass of the Higgs boson. Previous studies from the Large Electron-Positron collider operated by the European research organization CERN, along with indirect constraints from both the LEP and Tevatron experiments, had indicated that the mass of the Higgs could lie anywhere between 114 and 185 billion electronvolts (GeV).

The LEP shut down in 2000 to make way for CERN’s powerful Large Hadron Collider beneath the Swiss-French border, which debuted in September 2008.

Eight years of data from the Tevatron experiments now narrow the particle’s mass window, indicating with 95 percent certainty that the Higgs cannot have a mass between 160 and 170 GeV. That means the Higgs could only range between 114 and 160 GeV, or between 170 and 185 GeV.

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The 6,000-ton CDF detector, about the size of a three-story house, is one of two experiments at Fermilab’s Tevatron particle accelerator that has narrowed the allowed range of mass for the Higgs particle. CDF studies the debris created when hundreds of trillions of high-energy protons and antiprotons smash into each other.

Credit: Fermilab

The result not only restricts the possible masses that the Higgs can have, but also demonstrates anew that the Tevatron experiments, known as DZero and CDF, may actually find signs of the Higgs, says CDF scientist Rob Roser of Fermilab. “We now have the tools, techniques and luminosity at which to observe the Higgs,” he says.

According to the highly successful standard model, which describes all the forces in nature except gravity, all elementary particles were born massless.  Interactions with the proposed Higgs field would slow down some of the particles and endow them with mass. Finding the Higgs — or proving it does not exist — has therefore become one of the most important quests in particle physics.

Although the Large Hadron Collider was built to find the Higgs and will ultimately collide particles at seven times the maximum energy of the Tevatron, electrical problems forced the LHC to close for repairs soon after it opened. It won’t resume operations until this fall, and the accelerator isn’t expected to collide particles at its very highest energies until 2010.

Those delays have brought the DZero and CDF experiments, which have recorded hundreds of trillions of proton-antiproton collisions produced by the Tevatron, back into the spotlight. The Tevatron experiments “have a chance of scooping CERN and in a way I hope they do,” says physics Nobel Laureate Steven Weinberg of the University of Texas at Austin.

Because the Higgs particle can’t be detected directly, physicists must sift through the ashes of the miniature fireballs created when subatomic particles crash into each other at high energies. If the Higgs exists, it could decay into muons, into electrons paired with neutrinos or into jets of quarks. Because other elementary particles decay into these same particles, researchers have to analyze many trillions of events to look for a tiny statistical excess or deficit that might indicate the presence or absence of the Higgs.

In the range between 160 and 170 GeV, “we see fewer particles than what we would expect to get if there were a Higgs,” says Roser. He and his colleagues now plan to expand their analysis to energies just above and below this newly excluded range.

“Even if we were able to say something exciting about [the Higgs], it doesn’t really diminish what the LHC is going to do,” notes Roser. Researchers would still want to better characterize the particle, a job for the LHC. He also emphasizes that each of the 600 plus members of the Tevatron experiments are also participants on the LHC, muting the sense of competition.