LHC Has Found a Bump: Exotic Physics or Just Noise?

Science at the Large Hadron Collider is starting to wind down ahead of a scheduled winter break after carrying out experiments at record-breaking energies. Needless to say, these are exciting times; physicists are in an unknown realm of discovery where physics ideas beyond the Standard Model are being tested. And this week, LHC scientists announced something peculiar in two of the collider’s experiments.

But this “something peculiar” could just be a glitch in the data. Or maybe it’s not. Regardless, a tiny “bump” in datasets from two detectors has caused a buzz.

Conditions of the Big Bang

Before we go neck-deep into what the LHC has (or, more likely, hasn’t) found, we need to quickly understand how the largest experiment ever devised my humankind discovers new particles and new forces.

On Dec. 15, LHC collaborators met at CERN (the European Organization for Nuclear Research), the laboratory that manages the LHC located just outside Geneva, Switzerland. This was the first major meeting since the particle accelerator was upgraded earlier this year to accommodate higher energy collisions — a new phase called “Run 2″. The LHC is now accelerating particles around its 17-mile circumference ring of supercooled electromagnets at 13 teraelectronvolts (TeV) — an energy nearly double that of the energies physicists used to discover the Higgs boson in 2012.

Around the ring of electromagnets, several experiments are housed. These experiments are huge, building-sized detectors that are highly sensitive to finding particles that are generated after two counter-rotating “beams” of hadrons (such as protons or heavy ions, like lead nuclei) are forced to collide. These counter-rotating beams are traveling at relativistic speeds, so when they smash into one another, for the briefest of moments, the conditions that the universe hasn’t seen since the Big Bang are created.

The Big Bang is the genesis of the universe; all energy in the universe was unleashed from an infinitely dense singularity nearly 14 billion years ago. From this energy, as the universe cooled, a zoo of subatomic particles condensed to form the matter we know and love in the modern universe.

By recreating the conditions of the Big Bang in the LHC, physicists are able to peel back time and see for a very brief moment what primordial particles can be created by Nature, thereby testing physics theories on what particles are possible in our universe. In the case of the Higgs boson, physicists needed huge energies to produce the massive particle — “weighing in” at a mass of 125 gigaelectronvolts (GeV). The discovery of the Higgs confirmed that the Standard Model of physics (a quantum recipe book of sorts) correctly described all known particles and forces in the cosmos.

A Mysterious Bump?

Physics didn’t simply “end” with the confirmation of the Higgs, however. Many mysteries remain, not least why gravity was ominously not invited to the Standard Model party. Now physicists are looking beyond the Standard Model for answers — a realm known as “exotic physics.” It is in this realm that physicists hope to reveal evidence for dark matter particles, extra-dimensions, the possibility of supersymmetry, the hypothetical graviton and other stuff that we haven’t even thought of yet.

So, inside the same two LHC experiments that made the Higgs boson discovery in 2012 comes a signal that, albeit weak, has caused a minor stir.

Now that the LHC is smashing particles together at the highest energies ever attained, there’s hope that we may start glimpsing some exotic physics that, so far, are only hypothetical ideas. Like a photographic film that slowly collects photons to produce a photograph, the LHC’s detectors must carry out months or years of experiments to develop a clear picture. As we’re only a few months into this high energy experimental run, any results or signals will likely be blurry, but according to Run 2 preliminary results from the CMS and ATLAS detectors, a very slight bump in energy at around 750 GeV has been spotted. What could it be?

“We’ve been working round-the-clock to understand and triple-check our numbers, and (Dec. 15) was the culmination of the year’s worth of work by thousands of people,” said particle physicist James Beacham, a post-doctoral research fellow with the Ohio State University, in an interview with Discovery News.

Beacham is based at CERN and working on the ATLAS experiment to seek out “diphoton” signals in the huge quantities of data flowing from the massive detector.

Basically, when new particles are produced by high energy collisions, like the Higgs boson, they tend to decay very quickly. As they decay, they produce other particles that may be detected by LHC experiments.

The signature of this signal can reveal a fingerprint of the particle that decayed — in this case the excess could be caused by pairs of photons (diphotons) with an energy of 750 GeV. After more and more data are collected from billions and billions of collisions, small and unexpected bumps in collision data may start to rise from the noise, above what would be predicted by the Standard Model. These bumps are known as “excesses” and they can signal the production of new and exotic particles.

“The diphoton search, the one that has the most significant excess, is interesting because it could possibly discover things like exotic Higgs bosons or gravitons (the as-yet-undiscovered particles of gravity),” said Beacham. “Both of these discoveries would be revolutionary, because they’d be concrete evidence of physics beyond-the-Standard-Model, something we’ve never seen.”

The size of the bump is indeed tiny and may well wash away as more collision data is added, but the thing that makes this statistically tiny event interesting is that another detector, the CMS, has also detected a tiny signal in exactly the same 750 GeV energy range.

Although the signal is most likely noise at this early stage, physicists will of course be hoping for something exotic. But as cautioned by LHC physicists, even if this signal does turn out to be real, it could represent the presence of something decidedly un-exotic, like a more massive Higgs boson.

More Data Needed

As interesting as these matching bumps may be, it’s only the tiniest of hints that there’s something really there and Beacham is very clear, pointing out that the take home message is that "we need more data."

“When we saw this tiny hill in the diphoton mass spectrum in ATLAS we’re like, ‘Hmmmmm…‘ and then we instantly started poking it with our most ruthless experimental sticks, as usual, to see if it would withstand scrutiny. After poking and prodding (e.g., ruling out detector problems, multiple-checking the statistical methods) it was still there. But, again, the ‘it’ is just a slight uptick that, statistically, is just a hint,” he said. “We will have to remain on the edges of our seats for a few more months to years.”

His LHC colleagues agree: “It’s interesting because we did not expect it, and both experiments are seeing something in roughly the same place,” Jim Olsen, of Princeton University who works on the CMS detector, told Symmetry Magazine. “However, it’s not a discovery. It could be the first spark of a discovery, but we need more data before we know what it means -- if it means anything at all.”

Read more at Discovery News