Antineutrinos and 'sterile' neutrinos: IU scientists contribute to new advances in particle physics

Indiana University's place on the front lines of research on the mysterious subatomic particles known as neutrinos was on display June 4 to 9 at the 28th International Conference on Neutrino Physics and Astrophysics in Heidelberg, Germany.

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IU physicist Mark Messier, right, tours the space used to house the 14,000-ton NOvA detector with former Fermilab deputy director Young-Kee Kim in 2012. Messier is a spokesperson on the NOvA experiment. Photo courtesy Fermilab

Physicists from across the globe heard presentations about two developments in the field with connections to IU. The presentations concerned the first report of data onantineutrino oscillations from the NOvA experiment and evidence for a fourth neutrino type from the MiniBooNE experiment. IU scientists play lead roles on both these projects based at the Department of Energy's Fermilab in Illinois.

Neutrinos are subatomic particles without charge that barely interact with matter. There are currently three known types of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. The particles are unusual since they transform -- or "oscillate" -- between types as they travel over long distances.

Mark Messier, a professor in the IU Bloomington College of Arts and Science's Department of Physics who is attending the conference in Germany, has served as a lead scientist on the NOvA experiment since 2006, along with dozens of colleagues from the IU departments of physics, astronomy and chemistry who played a role in the design and construction of the experiment, including its central component: 9,000 tons of "liquid scintillator" used to detect neutrinos.

Rex Tayloe, also a professor in the IU Bloomington Department of Physics, served as the first project manager on the MiniBooNE experiment in 2000. Members of his lab and others at IU have spent over a decade traveling between Bloomington, Indiana, and Batavia, Illinois, to maintain the experiment and monitor data remotely.

Neutrinos are important because they play a major role in researchers' ability to understand the fundamental particles that make up everything in the universe, and how those particles relate to each other. Because neutrinos are so numerous, they influence how the universe evolves and may have been key to tilting the balance in favor of matter over antimatter in the first instants after the Big Bang.

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Rex Tayloe. Photo by Eric Rudd, IU Communications

"Much of what we don't know about neutrinos we can learn by comparing the oscillations of neutrinos to antineutrinos, but until now we haven't been able to produce a sizable sample of antineutrinos to study," Messier said. "With these results from NOvA, this window is now open."

The NOvA experiment is able to detect antineutrinos in part due to the powerful neutrino source at Fermilab and the experiment's enormous 14,000-ton detector, located 500 miles from Fermilab in northern Minnesota. This large distance -- the largest in any experiment using neutrinos created in a laboratory -- helps reveal differences between neutrinos and antineutrino oscillations.

Antineutrinos are the antimatter counterpart of neutrinos, Messier added. All fundamental particles -- protons, neutrons, electrons, quarks and neutrinos, among others -- appear in nature in both matter and antimatter forms.

"A question that has perplexed scientists is why the universe has matter in it when the laws of physics we know about show no preference for matter or antimatter," said Erica Smith, a postdoctoral researcher in the IU Bloomington Department of Physics and member of the NOvA project, who is also attending the conference in Germany. "Subtle differences between neutrinos and antineutrinos might be part of the explanation."

Similarly, Tayloe said the evidence for the existence of a fourth type of neutrino -- a "sterile neutrino" that interacts even more weakly with other matter in the universe than currently known neutrino types -- could help answer some questions about another poorly understood phenomenon in the universe: dark matter, an invisible substance that some estimate could comprise as much as 80 percent of the universe.

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The 14,000-ton NOvA neutrino detector in Ash River, Minnesota. Photo courtesy Fermilab

"A sterile neutrino would mean that there is a new 'sector' of particles out there that interact differently than the ones we know in the standard model of particle physics," Tayloe said. "That is a big deal, as it may hold the answers to questions like 'what is the dark matter' and other things related to the formation and evolution of the universe."

Many physicists from the MiniBooNE experiment  argue the project's data suggests a fourth neutrino type. That's because the experiment detected more neutrinos than expected from the experiment's neutrino source, which could be the result of unexpected oscillations, implying a fourth type of neutrino. The team's recent analysis also rules out many other possible explanations for the unexpected data.

Although neutrinos were first proposed in 1930, they weren't detected until the 1950s, and the first solid evidence for their oscillation wasn't produced until 1998. Since then, Messier said physicists have been learning to build neutrino beams intense enough to study the particles in greater detail.

Tayloe added that the result presented in Germany will surely result in new discoveries, including both greater insight into antineutrinos and the potential discovery of new neutrino types.

"An explanation for the data could be an extra, fourth neutrino type -- or there might actually be three new neutrino types," Tayloe said. "Other explanations, more exotic than sterile neutrinos, are also possible. There's still a lot left to discover."