Hope for New Particle Fizzles at the LHC

For months, the world of physics has been abuzz with rumors about a potential new subatomic particle that could revolutionize our entire view of physics. But new results presented today by physicists from the Large Hadron Collider (LHC) today have, for now, quashed the revolution. 

The first hints of a new particle appeared in December 2015, when two independent experiments at the LHC, ATLAS and CMS, each announced the same tantalizing quirk in their data. Both experiments smash together protons at nearly the speed of light, searching for new fundamental particles produced by the enormously energetic collisions. When they ramped up to their highest energies yet, the two experiments detected a mysterious signal: more pairs of photons with a combined energy of 750 giga-electron volts (GeV) than expected.

This “diphoton bump” was not a prediction of the Standard Model of physics—a rigorously tested and profoundly successful theory forged in the 1970s that incorporates all known fundamental particles and forces. Despite its success, however, the Standard Model does not explain what lies at the hearts of black holes, the nature of dark matter and dark energy, the quantum behavior of gravity, and other deep mysteries of the universe. With their shared diphoton bumps, ATLAS and CMS appeared on the verge of peering into physics beyond the Standard Model’s musty confines. Within weeks, the little bump had inspired hundreds of speculative papers by theorists. “At the LHC, physicists are looking very intensively for new particles and new laws of physics so it’s easy to get excited about something that seems very convincing,” says Michael Peskin, a theoretical physicist at Stanford’s SLAC National Accelerator Laboratory.

Whatever produced the diminutive diphoton bump didn’t neatly fit into any theory. Many scientists suggested that the bump was produced by a heavier cousin of the Higgs boson, another particle that similarly showed up as an eyebrow-raising blip in the data about four years ago. Others suggested that it could be a kind of dark matter particle, or even the vaunted graviton, the predicted carrier particle for gravity itself.

But as scientists at the LHC started collecting more data this year, the 750 GeV diphoton bump started disappearing. Now, after analyzing nearly five times the amount of data that they had last year, ATLAS and CMS physicists have watched the bump diminish to statistical insignificance. Presenting at the International Conference on High Energy Physics in Chicago, particle physicist and ATLAS spokesperson Dave Charlton said that when looking at all the data, the 750GeV signal now only has a significance of 2 sigma, which is much less than the 5 sigma (or 1 in 3.5 million chance) that is needed to confirm a new discovery in physics. Simply put, the diphoton bump was a false alarm. “It is a bit surprising that we saw the fluctuation on both instruments but it was just that—a fluctuation or statistical fluke,” said Charlton.

Seeing anomalies in the data is not uncommon at the LHC. The collider crashes so many protons together and churns out so much raw data that occasionally finding extra pairs of photons in the wreckage was bound to happen. “If you conduct many, many searches you come across these kinds of coincidences,” says Guy Wilkinson, a member of the LHCb collaboration.

Although the diphoton bump has now evaporated under closer scrutiny, researchers remain optimistic that the LHC will still lead them to new physics beyond the Standard Model. The multibillion-dollar project has years of operations left during which it will produce far more data for physicists to parse for elusive new particles. “We would have been very lucky if we found something, some new phenomenon or some new state of matter at this early stage,” says CMS physicist Tiziano Camporesi. “But we have to be patient.”



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