A lighter Higgs, but chase continues

Target narrows with new estimated mass for elusive particle

In the hunt for the Higgs boson, the world’s most powerful particle collider has tightened the net. New data collected this year by CERN’s Large Hadron Collider near Geneva narrow the range of allowable masses for the hypothetical particle, whose existence would confirm the mechanism thought to give mass to other particles.

An actual collision between two protons produces showers of particles whose paths and energies are recorded by the Large Hadron Collider’s CMS detector. This spray of electrons (thick red lines) and muons (thin red lines) looks similar to what would be expected from the Higgs, but could also be a background fluctuation. L. Taylor and T. McCauley/COPYRIGHT CERN 2011

To fit with the standard model, the cornerstone of modern particle physics, the Higgs must now be lighter than 145 billion electron volts, or GeV. Team members from LHC’s ATLAS and CMS experiments presented their results August 22 in Mumbai, India at the International Symposium on Lepton Photon Interactions at High Energies.

This new limit goes beyond previous results from the Tevatron at the Fermi National Accelerator Laboratory in Batavia, Ill. — which directly excluded 156 to 177 GeV by looking for debris left behind when the Higgs breaks down, and indirectly ruled out masses above 185 GeV using theoretical calculations and measurements of other particles.

“We’ve now confirmed with direct searches that the mass of standard model Higgs, if it exists, is light,” says CERN’s Fabiola Gianotti, a spokeswoman for ATLAS.

Even as it runs out of room to hide, though, the Higgs is still playing hard to get. Faint hints of the Higgs that turned up at the LHC in July — particles with energies that could indicate a lighter Higgs — have only grown fainter in the new LHC data. No statistically significant signs of the Higgs have appeared in the rest of the remaining mass range.

A lighter Higgs is harder to find at the LHC. At lower energies, its signature tends to fade into the background. But CERN physicists still expect to discover or rule out the existence of such a particle in the next two years.

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