Unraveling neutrino oscillations with MINOS

This figure shows the allowed region for the neutrino oscillation parameters represented by the area inside the contour. The black contour is the result from MINOS using its combined data set of beam and atmospheric neutrinos and antineutrinos, where the star corresponds to the best fit to the data. We compare this with what we would get by analyzing the data set from beam neutrinos only, shown in red.

Neutrinos are among the most mysterious particles that make up the universe, and they are not very easy to study. The three types can change from one to another in a quantum phenomenon known as neutrino oscillations.

In the MINOS experiment, we are able to measure these oscillations by producing a beam made of muon neutrinos (the NuMI beam) and detecting it in two different locations: at the near detector, located at Fermilab, and at the far detector, 734 kilometers away in Soudan, Minn. The large distance between the detectors gives the neutrinos a chance to change type, allowing us to observe neutrino oscillations.

The MINOS experiment has the special feature of being able to detect muon neutrinos and antineutrinos individually by separating the events each produces. Therefore we are able to study both muon neutrino and antineutrino oscillations, which are described essentially by two parameters: mixing angle and mass splitting.

Beyond that, we also use the far detector to detect neutrinos and antineutrinos created by interactions of cosmic ray particles with the nuclei in the Earth’s atmosphere. These are called atmospheric neutrinos and antineutrinos.

Several experiments have been measuring neutrino oscillations, helping us better understand this mysterious particle. For the first time, the MINOS collaboration has carried out a measurement by combining its two kinds of data: beam and atmospheric neutrinos and antineutrinos. We used the complete MINOS data set, accumulated over nine years of operation. The combined analysis has yielded the world’s most precise measurement of the mass splitting parameter for both muon neutrinos and antineutrinos. Furthermore, we compared results obtained for muon neutrinos and antineutrinos and found that they have practically the same oscillation parameters, providing more evidence that CPT symmetry is conserved in the neutrino sector. This is also the most precise comparison ever made between neutrino and antineutrino oscillation parameters.

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—Michelle Medeiros

Here we can see by how much the mass splitting of the antineutrinos differs from the neutrinos, where the dashed line represents the hypothesis that the parameters are equal. The dot is the best fit to the combined MINOS data set: Δm2antineutrino – Δm2neutrino = [0.12 (+0.24) (-0.26)] x 10-3 eV2, which is consistent with neutrinos and antineutrinos having the same oscillation parameters.