Bryce Littlejohn, assistant professor of physics, and his team have found that the impact of fission neutron energies cannot account for why fewer antineutrinos than expected are produced by nuclear reactors.

Their article about the work, “Impact of fission neutron energies on reactor antineutrino spectra,” was the top “Editor’s Suggestion” in the April 30 edition of Physical Review D, a leading particle physics journal.

It is another important piece of evidence in a growing pile telling scientists more about the possible existence of the “sterile neutrino,” the hypothetical particle first proposed in 1998.

Littlejohn is the principal investigator. His co-authors include Daniel Dwyer, physicist, Berkeley National Laboratory; Anna Erickson, associate professor, nuclear and radiological engineering, Georgia Tech; Andrew Conant, Ph.D. student, nuclear engineering, Georgia Tech; and Illinois Tech physics undergraduates Keith Hermanek and Ian Gustafson, who did the majority of the calculations for the paper.

The new paper builds on Littlejohn’s work last year with Senior Research Associate David Martinez and their Daya Bay collaborators finding that the “antineutrino anomaly” seemingly detected by Daya Bay researchers is instead more likely a modeling error.

The anomaly refers to the fact that scientists tracking the production of antineutrinos—emitted as a byproduct of the nuclear reactions that generate electric power—have routinely detected fewer antineutrinos than they expected. One theory is that some neutrinos are morphing into an undetectable form known as “sterile” neutrinos. Littlejohn, Martinez, and collaborators analyzed six years’ worth of data from Daya Bay—billions of data points—and found that indications of the theorized “reactor anomaly” likely arose not from sterile neutrinos but from incorrect predictions of how many antineutrinos nuclear reactors produce. If that work is confirmed, it may rule out forever the possibility of the sterile neutrino.

“Why, or in what way, is that model wrong?” Littlejohn asked. “One explanation that we investigated was that current models don’t handle Daya Bay reactors’ neutrons correctly. When neutrons run into atoms in a reactor’s fuel, those atoms split, or fission, which eventually gives us the neutrinos we look for at Daya Bay. We know that in all reactors, including Daya Bay, slow-moving neutrons and fast-moving neutrons produce different-looking neutrinos, and yet the current models only consider slow-moving neutrons. We thought this might be why the models predict the wrong number of neutrinos.

“We re-did the model calculations, this time correctly accounting for both neutron types (fast and slow),” he continued. “The net result after this change: There are still missing neutrinos. So, this small issue with the models still doesn’t explain why Daya Bay has ‘missing neutrinos.’

“While we didn’t identify the root cause of the ‘missing neutrinos,’ we did rule out one possibility that a lot of people in our community had been commonly discussing,” he concluded.