A joint Fermilab/SLAC publication

Higgs territory continues to shrink


The CDF and DZero experiments at the Fermi National Accelerator Laboratory have excluded a significant fraction of the allowed Higgs mass range established by earlier measurements. But they have not yet caught a glimpse of the elusive particle.

Scientists knew from previous measurements that the Higgs boson must weigh between 114 and 185 GeV/c2--the units that scientists use to measure the mass of particles. The new Fermilab result carves out a section in the middle of this range: the Higgs boson cannot have a mass in between 160 and 170 GeV/c2--if it exists at all.

Scientists from the CDF and DZero collaborations at DOE's Fermilab now have excluded a mass for the Higgs particle between 160 and 170 GeV/c² with 95 percent probability. A larger area is excluded at the 90 percent probability level. Earlier experiments at the Large Electron-Positron Collider at CERN excluded a Higgs boson with a mass of less than 114 GeV/c² at 95 percent probability. Calculations of quantum effects involving the Higgs boson require its mass to be less than 185 GeV/c².

Scientists from the CDF and DZero collaborations at DOE

The Higgs particle is the last missing piece in the theoretical framework known as the Standard Model of particles and their interactions. According to the Standard Model, the Higgs boson explains why some elementary particles have mass and others do not. Thousands of measurements at particle experiments around the world have led to the Standard Model over the last forty years, and precision measurements have confirmed it again and again. However, the Higgs particle--a keystone--keeps eluding detection.

The first observation of the Higgs particle is also one of the many scientific goals of the Large Hadron Collider experiments at CERN, which plans to record its first collisions before the end of this year.  The LHC will make particles collide more often and with greater energy than the Tevatron collider at Fermilab, but physicists familiar with the LHC expect that the first scientific results will not be available until the end of 2010.

In the mean time, the Tevatron might produce the first evidence for the Higgs.

"Fermilab's Tevatron collider typically produces about ten million collisions per second," said DZero co-spokesperson Darien Wood, of Northeastern University. "The Standard Model predicts how many times a year we should expect to see the Higgs boson in our detector, and how often we should see particle signals that can mimic a Higgs. By refining our analysis techniques and by collecting more and more data, the true Higgs signal, if it exists, will sooner or later emerge."

The Tevatron collider at Fermilab, four miles in circumference, accelerates protons and antiprotons close to the speed of light and makes them collide.

The Tevatron collider at Fermilab, four miles in circumference, accelerates protons and antiprotons close to the speed of light and makes them collide.

To increase their chances of finding the Higgs boson, the CDF and DZero scientists combine the results from their separate analyses, effectively doubling the data available.

"A particle collision at the Tevatron collider can produce a Higgs boson in many different ways, and the Higgs particle can then decay into various particles," said CDF co-spokesperson Rob Roser, of Fermilab. "Each experiment examines more and more possibilities. Combining all of them, we hope to see a first hint of the Higgs particle."

In the last two years the Tevatron has performed exceptionally well, and the 25-year-old machine continues to set numerous performance records, increasing the number of proton-antiproton collisions it produces. By the end of 2010, the Tevatron experiments will have recorded three times the number of collisions that scientists have used for the current analysis.

"We're looking forward to further Tevatron constraints on the Higgs mass," says Dennis Kovar, associate director of the Office of Science for High Energy Physics at the U.S. Department of Energy, which owns and funds Fermilab.

Watch a 2-minute video of Barbara Alvarez giving a tour of the CDF experiment and its search for the Higgs particle.

Watch a 2-minute video of Michael Kirby in the DZero control room, where he explains the on-going analysis of collision data in search of the Higgs particle.

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