LHC ends 2015 with a cliffhanger

The final Large Hadron Collider results of the year are in. Particles circulated in the LHC at CERN on December 13 for the last time in 2015, and, two days later, the two large general-purpose experiments, ATLAS and CMS, presented their results from LHC Run 2. Both experiments saw an unexpected bump, but scientists need more data to determine whether it’s worth getting excited about.

“It’s interesting because we did not expect it, and both experiments are seeing something in roughly the same place,” says Princeton professor Jim Olsen, who presented the CMS results during the end-of-year seminar at CERN. “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.”

LHC scientists search for new particles by comparing their actual experimental data to the predictions of the Standard Model, a well vetted model that describes elementary particles and their interactions. When experimental data consistently and significantly deviates from Standard Model theory, it could indicate the discovery of a new particle or process. The Standard Model leaves many questions unanswered, so physicists are searching for signs of Beyond-Standard-Model physics that might help them to answer some of those questions.

Scientists are currently exploring two possible explanations for the bump: It’s a normal statistical fluctuation and will disappear with more data, or it’s the early indication of a new particle with 750 times the mass of the proton. If it continues to grow in size and intensity, the bump could be evidence of a heavier cousin of the Higgs boson or a graviton—the theoretical particle responsible for gravity.

“Both of these discoveries would be revolutionary because they’d be concrete evidence of particles beyond the Standard Model, something we’ve never seen,” says James Beacham, a postdoctoral fellow at The Ohio State University. “But the take-home message is that we need more data.”

Olsen isn’t holding his breath; bumps like this tend to come and go.

“This new data has already helped smooth out a few bumps we’ve been watching since [the first run of the LHC],” Olsen says. “For instance, by summer both the CMS and ATLAS collaborations were seeing what looked like a particle with around 2000 times the mass of the proton. But we don’t see anything in this region from our new data. We can’t rule it out yet, but it looks like that bump might be going away.”

Still, not all bumps disappear. Around this time four years ago, physicists saw a small one at around 125 GeV. Six more months of data collection and analysis confirmed it was evidence of a Higgs-like boson.

These latest results come from the second run of the LHC, which began colliding particles at a record energy of 13 trillion electron volts (TeV) in June, compared with the maximum of 8 TeV attained during LHC Run 1 from 2010 to 2012. The amount of data on which the two experiments’ analyses are based is still limited—it’s about eight times less than what was collected during Run 1—and physicists need large volumes of data to be able to detect new phenomena. The experimentalists have nevertheless succeeded in producing numerous results. Each of the two experiments has presented around 30 analyses, about half of which relate to Beyond-Standard-Model research.

The presentations by ATLAS and CMS are available here.