UT researchers are hunting for an elusive subatomic particle after unexpected experimental results left the physics community puzzled. 

The particles in question are called neutrinos. They have no charge and very little mass, and they can pass through matter, including living tissue. These properties make neutrinos very difficult to detect. 

On Earth, most neutrinos are created from solar fusion reactions in our sun and cosmic rays reacting with the upper atmosphere. Scientists have long believed that neutrinos come in three different varieties of structures, or flavors, which they can oscillate between. 

Two neutrino experiments conducted over the past decade at Los Alamos National Laboratory and Fermilab generated results that contrasted previous scientific beliefs about neutrino oscillations, said Karol Lang, a UT physics professor and a spokesperson for one of Fermilab’s neutrino experiments.

“Neutrino oscillation is something that we now understand quite well, but we understand it within three neutrinos that we know of that exist,” Lang said. “To explain the results of these two other experiments, this is not enough. There has to be something beyond this in order to explain it.”

The data from the two experiments suggests the presence of sterile neutrinos, a theoretical fourth type of neutrino, according to Will Flanagan, a postdoctoral researcher working with Lang. 

According to Lang, few experiments have explored the possible properties of sterile neutrinos. In fact, there are many areas of research within this field that rely on data from these two experiments.

Lang and his team work on MINOS, the Main Injector Neutrino Oscillation Search, which is part of an experiment designed to investigate properties of neutrinos. MINOS operated from 2005 until June, and Lang and his team are now analyzing the data from the past ten years. 

In the MINOS experiment, a neutrino beam passes through a detector at Fermilab in Illinois and then travels 735 kilometers to an old iron mine in Soudan, Minnesota. The long distance is intended to give the neutrinos maximum time to change flavor. Once at Soudan, the neutrino beam hits another detector. By comparing the interactions of neutrinos between the two detectors, scientists can determine if the neutrinos changed flavor. 

Flanagan said the disagreement in the scientific community that these results have created is interesting.

“In the past, that’s always what’s led us to major discoveries,” Flanagan said. “We’re trying to say something complementary to this puzzle, because there are a lot of people in the camp that think we might have seen something, and a lot of people think it was just another part of the analysis that went haywire and we haven’t really seen anything.”

Tom Carroll, a physics graduate student working with Lang, said that because sterile neutrinos have weaker interactions than other neutrinos and can only be measured through neutrino oscillations, researchers need to make sure nothing else could explain their results. Carroll said other explanations could include ambiguities in the computer simulations that the researchers compare to their results.

“You have to check that you have all of your bases covered in terms of what it could have been,” Carroll said. “There are specific ways in which sterile neutrinos would affect neutrino oscillations, but since the sterile neutrino itself isn’t interacting, it’s a complicated process to try and tease it out to see if it was actually in that data.

Lang said the existence of sterile neutrinos could explain some big-picture physics questions, such as the evolution of the universe after the Big Bang. 

“It would be something new, yet undetected,” Lang said. “Every step of neutrino discoveries is associated with some puzzles, some hints, some potentially conflicting results. That’s why when we look at this situation … although it violates our understanding of nature under the three-neutrino paradigm, we shouldn’t dismiss it easily.”