Cecilia Lee

A novel neutrino study called the MicroBooNE experiment at Fermilab, spearheaded by Yale professor of physics Bonnie Fleming, found a lack of evidence for sterile neutrinos, small particles with no electrical charge and a mass close to zero. 

The 2015 Nobel Prize in Physics was awarded for discovering neutrino oscillations. As neutrinos travel through space, they shift between three flavors, or types, each of which has unique properties. The probability of these oscillations is a major area of study for the scientific community. In 2007, the MiniBooNE study at FermiLab found that oscillations occurred over 500 meters, which is a relatively short baseline according to Fleming, indicating that this evidence cannot fit within the three neutrinos in the standard model of physics. To account for this anomaly, a fourth neutrino was proposed, called a sterile neutrino, that alters the probability of oscillation. The MicroBooNE was launched in 2008 to confirm the existence of this fourth neutrino, but instead found a lack of evidence for its impact on neutrino oscillation. 

“The MicroBooNE experiment, whose results were presented last week, sits in the same beamline as the MiniBooNE experiment, but with a much more precise and capable detector,” said Fleming. “Our goal was to find exactly those same strange events, the appearance of electrons that would suggest electron-neutrino appearance, but we did not see them. We saw no excess electrons.”

These results raise the question: If not the sterile neutrino, then what is responsible for the excess events detected by MiniBooNE back in 2007? FermiLab and Fleming’s research group are continuing their search. 

So far only half of the collected MicroBooNE data has been analyzed. Additionally, though the MicroBooNE technology can reconstruct nine different interactions between neutrinos and various particles based on the electrons and photons the detector reads, only six have been tested so far. The other three may hold the key to neutrino oscillations, according to Fleming. 

Fleming’s collaborator, Yale postdoctoral researcher Jay Hyun Jo, sees the lack of a hint of the sterile neutrino as an opportunity. 

“Looking for an origin to MiniBooNE excess may lead to exciting new physics,” Jo said. Many alternative explanations include exotic particles, which are theorized to exist but have never been successfully detected under laboratory conditions. 

The MicroBooNE results are not merely significant due to their impact on the sterile neutrino hypothesis, but also because of the novel technology that is contained within the detector. The MiniBooNE detector was unable to differentiate between electrons and photons, but the MicroBooNE detector has this capability. 

First prototyped in Europe and its extended tracks first used in the US in Fleming’s lab back in 2007, the Liquid Argon Time Projection Chamber is revolutionary. The device is not only incorporated into MicroBooNE, but also into the 2029 Deep Underground Neutrino, or DUNE, experiment that will expand this neutrino oscillation detection project to a massive scale. 

“What we will do with the DUNE experiment is look at how much neutrinos oscillate from FermiLab to South Dakota and compare that with how much antineutrinos oscillate and hope to see a difference,” said Fleming. “In the lab we can take energy, and see that matter and antimatter pairs are created, an electron and an anti-electron, and then we can see that they join and annihilate back into energy. The problem is, that’s all we see. And if that had happened in the early universe, it would have been a beautiful symmetric process, but we would not have existed.” 

Due to researchers observing matter and antimatter cancelling each other out in the laboratory setting, there is no evidence for why matter exists. However, if neutrinos and antineutrinos are proven to oscillate differently, this would demonstrate a pairing of matter and antimatter in which the two do not cancel each other out. This could have significant implications showing why there is a preponderance of matter.

According to Hanyu Wei, a former postdoctoral researcher at Brookhaven National Laboratory and a member of Fleming’s team, “neutrinos are the second most abundant particles in the universe.” However, scientists have not been able to detect them until about 50 years ago. This is a new area of science that scientists in the field are just beginning to understand. 

Fleming summed up the significance of these neutrino experiments and their implications in investigating the existence of matter.

“We are trying to figure out if neutrinos are the reason we exist,” Fleming explained.

The DUNE experiment started construction at the Sanford Underground Research Facility in South Dakota in 2017.

Correction, Nov. 3: A previous version of this article wrongly stated that the Liquid Argon Time Projection Chamber was prototyped in Fleming’s lab. In fact, it was first prototyped in Europe.

VALENTINA SIMON