Tantalizing Data From the LHC Has Physicists Psyched

Late last year, two different instruments at the massive, subatomic particle-smashing Large Hadron Collider saw … something. No one knows what the so-called bump in the data was—it came from pairs of photons crashing into the detectors at the same time, with the same energy. Working backward from those kinds of crashes, physicists can infer things about the death and decay of larger particles. Usually.

This time, that wasn’t so easy. The new “di-photon excess”—unpredicted and unexpected—could point to a particle four times heavier than the next-heaviest particle, the top quark, and six times heavier than the famous Higgs boson. It could suggest the existence of a heavier relative of the Higgs, or maybe a graviton—the still-theoretical particle that conveys gravity. Or it could be something completely novel, a harbinger of new physics as yet undiscovered.

Or it could be nothing—a statistical fluke, a ghost rising out of the machine.

The LHC’s next run, in April, could provide an answer. But particle physicists turn out to be an impatient lot. As they wait for the new data, they have been working on the wiggle, posting hundreds of papers to the open-access site arXiv.org attempting to interpret this still-statistically-insignificant data. Is it a new particle? What would that mean? What would it be like?

Sure, the physicists could just wait to see whether it’s a spirit or if it has substance. But they don’t want to.

The Shock of the New

“This is exactly the spirit of theoretical physics,” says Gian Giudice, a theoretical physicist at CERN and author of one of the papers that generated the most smash. “You take the data at your disposal, and then you start thinking if it fits with some of your ideas about the universe.”

Then you figure out how to test those ideas. You get more data. You see how it meshes with the first ideas and then maybe change them. “On and on,” Giudice says, “until you have a credible story about our universe.”

Still, physicists don’t jump on every anomaly. Weird lumps and bumps appear in data all the time, and physicists don’t rush to write papers, says Michele Redi, a theoretical physicist at the National Institute of Nuclear Physics in Florence and one of Giudice’s collaborators. “But this one smelled different,” he says.

Physicists want nothing more than for the seams of the Standard Model to split. It would let them build a new version.

For one thing, hints of the bump showed up in data from two different instruments. That’s at least suggestive that it’s real and not some kind of artifact. But perhaps more importantly, if these results are real, they have the potential to overturn or at least extend the Standard Model, a framework developed in the 1970s that explains how particles interact. “The Standard Model of particle physics has been very successful in describing the interactions of all the particles we directly observe in nature, and also their interactions,” says Rob McPherson, a spokesperson for the Atlas instrument, one of the two detectors that caught the signal. “But there are several unanswered questions.”

The Standard Model is good, and so far much of experimental physics has confirmed its outlines. But it doesn’t explain everything: It has holes, and mysteries like dark matter, antimatter, and gravity fall right through them.

That’s why physicists want nothing more than for the Standard Model’s seams to split. That would give them the chance to develop a new version—and build a new understanding of the universe. After all, most of today’s theoreticians weren’t yet working in the 1970s, and they missed out on the excitement of the first time around.

The Race

For years, the chances of upending the Standard Model seemed slim. It looked like the LHC would yield a megaton of data about old physics and not a picogram of anything new. “To put it simply, it is a dream that nobody in our field had anymore,” says Redi. That changed with the December 15 bump. Maybe the new physics was right there, underneath it. Redi and his colleagues worked around the clock to dissect it and be first to press “publish.”

Mihailo Backovic of the Université Catholique de Louvain in Belgium also wrote a rush paper, “Di-Photon Excess Illuminates Dark Matter.” He knows it’s just an exercise in what-if. “Above everything, the reason why theoretical physicists do anything in physics is because it’s fun,” Backovic says. “It’s the only reason why we are in this field—to satisfy our childish fascination with the universe.”

Part of that fun, though, is the possibility of being the first one to discover something new. Most scientific discoveries—sorry, science—are workaday, incremental. Rarely (by definition) does a paradigm get shifted. And even more rarely does a shift come along publicly, with results just begging to be interpreted. If scientists waited for verification of the bump’s existence before theorizing, they would have no shot at being right first. “The ownership of an idea nowadays is determined by the day—maybe even hour—of the paper submission,” Backovic says, “so people feel they need to try and publish their work before someone else does.”

More on the LHC

When a colleague sent Backovic a chronological plot of how many papers he and his theorist friends had thrown up on the arXiv over time (as of March 10, there were 276), he dove into that data, too. “It struck me that the curve on the plot looked surprisingly regular, and it made me think about whether I could understand the shape,” he says. “So I started doing what physicists do: playing with data and trying to model it.”

On his daily commute, Backovic fiddled with statistics, trying to find a mathematical representation that would show how many papers appeared over time and predict how that would change in the future. He found he really only needed two assumptions: that both people’s interest in the di-photon excess and the number of ideas left to write about decrease as the days pass. “I thought it was very neat to show that something that is seemingly very complex, driven by human behavior, can be modeled by very simple math,” he says. (Well, simpler than the behavior of fundamental particles, anyway.)

Backovic projects that around 310 papers will appear before June 10. In the acknowledgments of his paper, he wrote, “I would like to thank the SNCB Belgian Railways for providing a comfortable environment on the trains where most of this work was conducted, as well as for frequent delays in the train system which provided the much needed additional time to complete the project.” He called his work “A Theory of Ambulance Chasing.”

And the odds of actually catching that ambulance? “I would put the chances that there is a new particle at about 20 percent,” he says.

Redi’s a little more optimistic. “It is very hard to say, and most people would refuse to give odds,” he says. “However, I like to play the game, and if I had to bet my own money I would give it 50 percent chances of being real.”

I would put the chances that there is a new particle at about 20 percent. Mihailo Backovic, physicist

Giudice refused to bet. He said he didn’t have a crystal ball and couldn’t see the future

McPherson, conversely, looks to the past. He’s seen a number of these beyond-Standard-Model “excesses” over the last couple decades. “So far, they have all receded with refined analysis and additional data,” he says. The Large Electron Positron Collider saw what physicists thought were hints of the Higgs. They got hyped up. That data was even better (more significant) than the latest bump, McPherson says, and it turned out to be random background fluctuations. The Higgs remained undiscovered for more than a decade.

In his paper, Backovic analyzed publishing patterns after eight other hype-heavy situations in physics. The initial paper spike didn’t signal future success. On his list were the BICEP2 observations supporting cosmic inflation, and evidence that neutrinos travel faster than light—both of which scientists later retracted. (After the time of the take-back, Backovic’s models no longer work well, because that whole interest thing fades fast.)

No matter what, Backovic says, the prep work they’re putting in is worth it. If the bump shrinks, they still benefited from the discussions and mental gymnastics. “If a physicist complains that this is a waste of time, they are probably in a wrong profession,” he says.

The relevant LHC instruments will spool back up in April. By the time summer arrives, the collider will have generated as much data as it did in all of 2015, and twice that by the end of the season. McPherson says they hope to have definitive reports in time for the International Conference on High-Energy Physics in August. Maybe the bump will rise above the noise and achieve the five-sigma significance that physicists require for something to be true. Or maybe the noise will gain ground over signal, and the ghost particle will turn out to be a fake, Scooby-Doo type ghost, a guy in a mask. And it would have gotten away with it, too, if it hadn’t been for those meddling physicists.