New clues of the existence of sterile neutrino from FermiLab’s MiniBooNE experiment

The inside of MiniBooNE (Photo Fred Ullrich of Fermilab)
The inside of MiniBooNE (Photo Fred Ullrich of Fermilab)

An article published on the arXiv server describes the observation of an excess of electron neutrinos detected by FermiLab’s MiniBooNE experiment. The results are consistent with regard to energy and order of magnitude with those obtained in the 1990s by the LSND experiment in Los Alamos. The most likely explanation is that it’s a proof of the existence of the sterile neutrino, a type of neutrino hypothesized but at least so far not found despite a lot of targeted research.

Neutrinos are particularly elusive particles because they have a very little mass and interact with other particles in a very limited way. Decades of experiments led to the conclusion that neutrinos are of three types, or flavors: electron, muon and tau, respectively associated with electron, muon and tau particles. A neutrino can change flavor, a phenomenon called oscillation.

To study these strange particles, various detectors have been built over the decades and various experiments tried to better understand their characteristics. The experiments conducted in a collaboration between CERN in Switzerland and the Gran Sasso National Laboratories in Italy were at the center of a case a few years ago with the speed of neutrinos that seemed faster than light until the instrument controls revealed an anomaly.

Another experiment, Liquid Scintillator Neutrino Detector (LSND) in Los Alamos, in the USA in the 1990s detected an anomaly consisting of an excess of electron neutrinos. This was considered an indication of the existence of sterile neutrino but other experiments failed to replicate those results. The LSND’s experience led to the construction of MiniBooNE (Mini Booster Neutrino Experiment), which now achieved similar results after twenty years.

According to the authors of the article, those results have a 4.8 sigma, a parameter that calculates the probability that they are correct and not the result of a random fluctuation that can occur when you’re dealing with elementary particles. 4.8 is a value that indicates a high probability of correctness and according to the researchers the combination of these results and those of LSND offers a 6.1 sigma, which means a probability of 500 million that this is a random fluctuation.

The problem of calculating the sigma parameter is complex and in any case insufficient to provide a final answer to the search for the sterile neutrino. A 5 sigma would satisfy scientists and would require only a verification of the correctness of the data used to calculate it. Instead, the 6.1 sigma claimed regarding the combination of the data of MiniBooNE and LSND requires more details to justify it.

Mathematics aside, it’s crucial to understand why only the LSND and MiniBooNE experiments provided indications of the existence of sterile neutrino while other neutrino experiments have always given negative results from this point of view. If sterile neutrino really existed, we’d have to update the Standard Model, a quantum field theory that describes the strong and weak interaction and electromagnetism, three of the four fundamental forces.

Joe Lykken, FermiLab’s deputy director and chief research officer, stated that next year will be exciting for neutrino research with the possibility of understanding whether or not sterile neutrino exists. That’s because in addition to MiniBooNE, research will be carried out with the recently updated ICARUS detector and the new Short Baseline Near Detector (SBND) detector.

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