Rather, scientists measure their properties by observing the groups of particles, collectively called final states, that are produced on the rare occasions that a neutrino interacts with the matter in a detector. Neutrinos themselves leave no signals in particle detectors. The MicroBooNE and MiniBooNE experiments are located along the same neutrino beamline at Fermilab in Illinois. The MicroBooNE Collaboration has now performed three analyses of their data to try to determine the nature of the signal MiniBooNE detected. But the MiniBooNE detector picked up a 𝜈 e-like signal that suggests exactly that switch (see Viewpoint: The Plot Thickens for the Fourth Neutrino). For the MiniBooNE experiment, such calculations are clear: if the picture of three-neutrino oscillation is complete, the 𝜈 𝜇’s produced at the start of the beamline should not oscillate into 𝜈 e’s.

In a given experiment, the distance and energy range are set, and one can calculate the oscillation probability using measured values for the neutrino mass differences and mixing angles. This probability depends on the distance the neutrino has traveled, the energy of the neutrino, the differences in mass between the neutrino mass states, and the mixing angles that characterize the linear combinations of states. As a neutrino propagates, the phases of its mass states change, altering the probability of detecting it in one flavor or another. Rather, each is described by a linear combination of the three neutrino mass states. They find no such anomalous signal, but they can’t yet rule out the possiblilty that new neutrinos might be out there. The MicroBooNE Collaboration has now further explored this exciting possibility. In 2021, the MiniBooNE Collaboration reconfirmed earlier observations of an anomalous signal that could be incompatible with our understanding of the oscillation of the three known neutrino flavors. Today, neutrino-oscillation experiments can measure the “disappearance” and/or “appearance” of neutrinos as they switch flavors.

This behavior was first seen in the 1960s when scientists measured fewer neutrinos from the sun than they expected, but it took decades of experimental work to prove conclusively that these “missing” neutrinos had changed flavor on their journey to Earth. This possibility has been suggested by the results of experiments looking at neutrino oscillation-a quantum-mechanical phenomenon that involves a change in the flavor of a particle as it travels. But physicists think that there could be more. There are three known flavors of the elementary particle known as a neutrino: 𝜈 e, 𝜈 𝜇, and 𝜈 𝜏. The colors represent the amount of energy deposited in the detector, with red representing higher-energy deposits. MicroBooNE Figure 1: When an electron neutrino ( 𝜈 e) interacts with the MicroBooNE detector, it produces an electron and a proton whose paths the detector precisely tracks.

Neutrino symbol