For particle physicists, “expect the unexpected” is more than just a catchy tagline.

Duke scientists on the Large Hadron Collider’s (LHC’s) ATLAS collaboration are on the hunt for hints of the unexpected: new, undiscovered particles or forces that could point to theories beyond the remarkably accurate, yet clearly incomplete, Standard Model of physics.


The Duke physics team at CERN this summer, gathered in front of a model of one of the LHC’s superconducting electromagnets. (Left to right: Ifeanyi Achu, Emily Stump, Elisa Zhang, Hannah Glaser, Wei Tang, Spencer Griswold, Andrea Bocci, Minyu Feng, Shu Li and Al Goshaw).

But the tsunami of new data coming out of the LHC’s current run, which began May of this year, has yet to provide any promising clues. Notably, at the ICHEP conference in Chicago, ATLAS collaboration members presented new results showing that an intriguing “bump” observed in 2015 data — speculated to be the first evidence of a completely new particle six times the mass of the Higgs — was likely just a statistical fluctuation in the data.

“It was quite amazing,” said Duke physics professor Al Goshaw, a member of the ATLAS collaboration. “With this new data there should have been a very clear signal, and there is nothing. It’s just absolutely gone.”

Goshaw has spent much of the summer at CERN, leading a team of undergraduate and graduate scientists crunching the numbers on the new data. Undeterred by the results presented in Chicago, he says the Duke team is still hard at work searching for other massive new particles.

“Our plan is to take the full data set collected in 2016 and extend the search for a new force-carrying particle up to much higher energies,” Goshaw said. “The search will go up to about 25 times the mass of the top quark or 35 times the mass of the Higgs.” They aim to have the results of this analysis ready by early 2017.

Why all the interest in tracking down these massive new particles?


Particle and energy spray recorded following a high-energy proton-proton collision event at the LHC in May. (Credit: CERN)

Goshaw says there are a myriad of alternative theories to the standard model, so many that trying to test specific predictions of individual models would be prohibitively time-consuming.

“But there is one prediction which they almost all make, and that is that there should be additional massive particles beyond those contained in the standard model,” Goshaw said. “So a generic way to search is to look for the new forces which are indicated by a force carrier, a massive new particle.”

The new data, collected at higher energies than the 2010-2012 run and with higher “brightness” or luminosity than the 2015 run, gives physicists the best chance yet of spotting an elusive new particle.

However, it’s not always looking at a plot and looking for a little bump, Goshaw says. Physicists, including the Duke team, are also utilizing the new data to perform highly precise tests of the standard model.

“The precision tests are really trying to find cracks in the standard model,” Goshaw said. “There could be particles that are so massive that we cannot detect them, but they may appear as subtle deviations in standard model predictions.”

But for now, the tried-and-true still holds. “It is quite extraordinary that, with these beautiful tests, everything is still described by the standard model,” Goshaw said.

Kara J. Manke, PhD

Post by Kara Manke