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Favored Higgs hiding spot remains after most complete search yet

The Standard Model Higgs boson is excluded at a 95 percent confidence level everywhere the thick black line drops below the red line. Image: ATLAS and CMS experiments

The CMS and ATLAS experiments at the Large Hadron Collider have backed the Standard Model Higgs boson, if it exists, into a corner with their first combined Higgs search result.

The study, made public today, eliminates several hints the individual experiments saw in previous analyses but leaves in play the favored mass range for the Higgs boson, between 114 and 141 GeV. ATLAS and CMS ruled out at a 95 percent confidence level a Higgs boson with a mass between 141 and 476 GeV.

The new result combines eight studies of predicted decays of the Higgs boson using data the experiments collected up to July. Physicists expected to be able to rule out an even wider mass range, between 125 and 500 GeV, based on the amount of data used and the sensitivities of the different search modes. But small excesses that could be the result of statistical fluctuations or could indicate the presence of a hidden particle reduced the range of masses that could be excluded.

“I think it could be an interesting message the data is telling us,” said physicist Eilam Gross of the Weizmann Institute of Science, who shares leadership of the ATLAS experiment’s Higgs group. “Any discovery starts with the inability to exclude.”

Several related measurements indirectly suggest a Standard Model Higgs boson exists at the lower end of the mass range.

Almost a year of work, more than 50 meetings and plenty of diplomacy went into calculating the LHC experiments’ first combination of Higgs search results. Experts from both collaborations worked together for more than seven months to devise the combination procedure and for about one month to come up with the combined result. Then the two collaborations, each a few thousand members strong, took about a month to review and approve it.

“This is really a landmark achievement,” said CMS physicist Vivek Sharma of the University of California, San Diego, co-leader of the Higgs combination group. “Combining LHC Higgs search results is not a straightforward process. There’s a lot going on under the hood.”

Since July, the two experiments have accumulated two to four times as much data. By the end of 2012, they hope to at least double that.

The end of the Standard Model Higgs search is in sight, said ATLAS physicist Bill Murray of the U.K. Science and Technology Facilities Council, co-leader of the Higgs combination group. “This year’s data will probably give us the answer, but next year we will be able to announce it firmly,” he said.

So far, the ATLAS and CMS experiments have not yet decided to produce a second Higgs combination based on results from the full 2011 dataset. The two experiments are still in competition; they might wait to see what they can accomplish on their own before working together again.

“This was a chance to go through the process when the results were uncontroversial,” Sharma said. “Now we’re in discovery mode. The next time won’t be so calm.”