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Evidence for new particle vanishes: LHC data find no signs of a relative to the Higgs boson.

A fresh analysis of data from the particle collider that delivered the Higgs boson has dashed physicists' hope that another new particle had emerged from the subatomic shrapnel.

"We've learned that there's no obvious Godzilla particle hiding with the Higgs," says Tim Tait, a theoretical physicist at the University of California, Irvine. "Now we're going to have to look for more subtle signs of new particles." Discovering particles beyond the Higgs could help physicists understand mysterious components of the universe, such as dark matter.

In July 2012, physicists at the Large Hadron Collider, or LHC, near Geneva, announced the discovery of the long-sought Higgs boson (SN: 7/28/12, p. 5). The Higgs was the last particle to be detected among those predicted by the standard model of particle physics.

CMS and ATLAS, the two LHC detectors that uncovered the Higgs in the debris of proton collisions, didn't observe the particle directly. Instead, they analyzed the shrapnel produced when a Higgs decays, less than a billionth of a trillionth of a second after it flashes into existence. Higgs bosons can decay in multiple ways; in one scenario, a Higgs transforms into a top quark or a W boson, either of which would in turn immediately break up into two long-lived, detectable photons.

Among the multiple Higgs decay products that helped prove the particle's existence, photons were the only ones that appeared more often than the standard model predicted. This intriguing photon excess suggested that a Higgs boson could decay into a third, unknown particle that would then break up into more photons.

"We were all a little hopeful," says Pierre Savard, a University of Toronto particle physicist with the ATLAS experiment. At least 100 researchers offered ideas for particles that could produce the photon results, Tait estimates.

But when CMS updated its data last year, the photon excess disappeared. And in a study posted August 29 at arXiv.org, ATLAS physicists report that the excess signal has disappeared in their data, too.

"We have no strong hints of new physics," Savard says.

While Tait admits it would have been exciting if the photon excess had held up, he stresses that there is plenty of opportunity for discovery in the future. Many theories positing new particles and forces predict only subtle deviations in LHC data from the standard model--effects that are too small to be weeded out until even more data are in hand.

And much more data are on the way. The LHC was shut down in February 2013 for upgrades, but it will resume particle smashing in spring 2015 (SN Online: 6/23/14). The improved collider will slam protons together at higher energies, perhaps revealing heavier particles inaccessible to the first-generation LHC. Plus, collisions will occur more frequently, providing more data to help physicists separate signal from noise. "To find new particles, we want to give it everything we've got," Tait says.

Caption: The ATLAS detector, shown during construction in 2007, has found no trace of a suspected new particle in addition to the Higgs boson.

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Title Annotation:ATOM & COSMOS
Author:Grant, Andrew
Publication:Science News
Geographic Code:1USA
Date:Oct 18, 2014
Words:519
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