MicroBooNE likely has ruled out possibility of ‘sterile neutrinos’

Professor Andrew Furmanski of the School of Physics and Astronomy is a member of an experimental collaboration which has eliminated the possibility of a theorized possible fourth neutrino flavor, the ‘sterile’ neutrino with 95% confidence. MicroBooNE neutrino experiment at Fermilab published a paper in Nature recently showing that their two-beam, liquid argon detector has set the most stringent limits yet on the possibility of a fourth neutrino flavor.  

The so-called “sterile” neutrino was theorized to help “explain some weird experimental results going all the way back to the 90s,” Furmanski says. The three neutrino flavors, part of the standard model of particle physics, have been observed to leave the beam as one flavor and arrive in the detector as another flavor. This phenomenon is caused by quantum mechanical interference known as “oscillation.” There have been oscillations observed that do not correspond to existing theoretical models around neutrino oscillations, so theorists put forward the idea of a fourth flavor that doesn’t directly interact with other matter, but still somehow causes certain neutrino oscillations to go awry. 

MicroBooNE result showing an electron neutrino from the NuMI (NOvA) beam.

“For the last 20 years, people have been trying to either come up with some concrete evidence or rule them out in a way that’s more conclusive than previous experiments,” Furmanski says. “Microboone– kind of by chance– has two neutrino beams.” MicroBooNE is in the beamline of the Booster Neutrino Beam and it also lies very near to the beam used for the NoVA experiment in Northern Minnesota. 

“We call them beams, but they aren’t like a laser, more like a flashlight. So we see the edge of the neutrino beam that is pointed at Northern MN. They weren’t thinking about that thirty years ago when they set the NoVA beam up.” 

Two beams become important when one considers that there is the possibility that the anomalies in flavor change could be explained by a case where two flavor changes happen simultaneously, and cancel one another out, so it looks like there was no oscillation. With the second beam, there is no chance that the mix of particles would be identical. Oscillations that cancel each other out could not occur in both beams at the same time.

“MicroBooNE has been taking data since, and has published a bunch of measurements with the NOVA Beam, but this is the first time we’ve combined both beams in this analysis.” 

The results have already achieved 95% confidence that they’ve eliminated the possibility, but they hope to refine it even further by processing more of the data they’ve already taken, some 30% of their total is yet untouched and there is more analysis on the way.

“If there’s a sterile neutrino, we can see an electron neutrino and a muon neutrino, but we can’t see tau neutrino,” Furmanski says. “If there really is a sterile neutrino, some of them should be switching to sterile and staying there. You should see them changing. One of my grad students, Nikki Pallat, is working on that analysis.” 

The two other experiments in the Short Baseline Program may also provide even more data to assist in better understanding neutrino oscillations. To this end, the Program is setting up a data sharing program, so that these three independent collaborations could do analysis on all three detectors. “The short baseline program lets you measure the same neutrino multiple times, you can see how they are changing. It will be a very conclusive test.” 

All of Furmanski’s work on MicroBooNE should be seen in the larger context of its importance as a proof of concept for a much larger future experiment, the Deep Underground Neutrino Experiment (DUNE) being installed in a former gold mine in South Dakota. DUNE will also be a liquid argon detector on a much bigger scale. When MicroBooNE started running ten years ago, it wasn’t obvious that these detectors would function and be stable. 

“MicrobooNE has done world leading measurements on sterile neutrinos. In a certain sense it was a test case for the much larger DUNE experiment to come.”

Professors Wilking, Pawloski, Heller and Marshak all of the School of Physics and Astronomy are part of the DUNE experiment. Furmanski’s group consists of Postdoc Miguel Hernandez Morquecho, and graduate students Nikki Pallat, Kate Hildebrandt, and Steven Metallo.