Evidence for new Higgs-related particle fades away

A fresh analysis of data from the particle collider that delivered the Higgs boson has dashed physicists’ sliver of 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, which holds galaxies together yet does not absorb, reflect or emit light.

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 had been the last particle yet to be detected among those predicted by the leading particle physics theory, known as the standard model.

CMS and ATLAS, the two gigantic LHC detectors that uncovered the Higgs in the debris created by two colliding proton beams, didn’t observe the particle directly. Instead, they homed in on the signature shrapnel produced when a Higgs decays, after it flashes into existence for less than a billionth of a trillionth of a second. Higgs bosons can decay in multiple ways, and in one scenario, a Higgs quickly transforms into one of two particles, a top quark or a W boson, either of which would in turn immediately break up into two long-lived, detectable photons.

Those photons created lots of excitement following the 2012 announcement. 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. Data from both CMS and ATLAS revealed this intriguing photon excess, which suggested that a Higgs boson could decay into a third particle, one unknown to physics, that would then break up into more photons.

“We were all a little hopeful,” says Pierre Savard, a University of Toronto particle physicist who co-leads the ATLAS experiment’s Higgs physics group. At least 100 researchers offered up ideas for hypothetical particles that could produce the CMS and ATLAS photon results, Tait estimates.

But as the LHC continued churning, the photon data started to look less unusual. Last year, CMS updated its data and found that the photon excess disappeared. Physicists still had ATLAS as a beacon of hope for a new particle — until now. In a study posted August 29 at arXiv.org, physicists working on ATLAS report that the excess photon 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 near future. Many theories positing new particles and forces predict only subtle deviations from the standard model in LHC data — 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.