"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.
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"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."
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Beacham is hopeful that we'll have significantly more data by next summer that could either strengthen this particular bump or flatten it out. High-energy physics is as much about statistics as it is building the most powerful particle colliders on the planet - deciphering signals needs to be statistically robust and this particular CMS/ATLAS excess only registers at the 2σ ("2 sigma") level. For something to be called a "discovery" the signal needs to hit 5σ, basically the "gold standard." Currently, this bump is considered nothing more than a statistical wiggle, but the fact that 2 detectors are seeing something rise out of the fog of collision data could be more than a coincidence.
So for now, we wait for the LHC to continue its job of colliding particles, spewing a firehose of data ripe for analysis. But first, a pause for LHC physicists to get some rest in the run-up to Christmas.
"CERN shuts down for the winter break in a few days, and I think a lot of us who've been breaking our necks playing with our new, shiny 13 TeV data (small bump or no) will sleep for a week," said Beacham.
For more details behind the CERN meeting on Dec. 15, check out the organization's news release.