When it comes to neutrinos, it’s best to expect the unexpected.
Previous Results of the Week have showcased a surprising difference between MiniBooNE electron neutrino appearance and electron antineutrino appearance results. In this special result, we present an analysis done by combining MiniBooNE and SciBooNE data to improve our understanding of a MiniBooNE analysis from 2009.
Previously, MiniBooNE looked for an excess of electron neutrino events in a muon neutrino beam over a short distance (0.5 km). Experimenters then conducted the same search using antineutrinos. While the tests were the same, the results were surprisingly different. The neutrino data is consistent with background, but the antineutrino data shows an excess of events consistent with the controversial 1990 results from the Liquid Scintillator Neutrino Detector experiment at Los Alamos National Laboratory.
If this observed difference is due to new physics, the new physics must be rather exotic. The most common explanation for these results uses the idea of sterile neutrinos, which physicists believe are neutrinos that do not have charged partners. Collaborators believe that as the muon neutrino travels, it will sometimes convert into a sterile neutrino, which then would convert into an electron neutrino. We expect that the sterile neutrino is only detectable from this reaction.
If sterile neutrinos exist, then the muon neutrinos should disappear, that is, some of the muon neutrinos will have converted to undetectable sterile neutrinos and the rate of muon neutrinos will be lower than we expect. Let’s say the muon neutrinos constitute a pie before baking. Disappearance is characterized by a missing slice of this pie, as some of the muon neutrinos have changed into sterile neutrinos, which we can’t see.
A previous search for the disappearance of muon neutrinos and muon antineutrinos two years ago compared MiniBooNE data to the predicted number of events at the detector. This is like counting the ingredients and examining the empty pie tin before baking, and then estimating the total pie weight and size after baking without looking at it directly.
Of course, this method is limited by our understanding of the initial number of neutrinos that reach MiniBooNE and the specifics of how they interact, that is, how well we know the ingredients beforehand .
Now, MiniBooNE has teamed up with the SciBooNE experiment to perform an improved analysis on the disappearance of muon neutrinos. SciBooNE, a dedicated cross section experiment shares the same neutrino target and flux as MiniBooNE, but was located in the same neutrino beam closer to the neutrino source. By adding the SciBooNE data to our analysis, we are able to measure the neutrino rate before the muon neutrinos disappear. This is like weighing the pie and inspecting it before baking, and is less dependent on our initial predictions.
The first joint venture of these two experiments observes no muon neutrino disappearance at 90 percent confidence level, which constrains models that require large amounts of disappearance. Our next step will be to look at muon antineutrino disappearance with both experiments, an important step to understanding the nature of new physics, if it exists.
— Kendall Mahn and Yasuhiro Nakajima