Neutrino scientists are currently trying to answer some exciting questions. How much do neutrinos weigh and why are they so light? How much do neutrinos change from one kind to another (called mixing) and why are their transformations so different from quark mixing? Do neutrinos mix differently from anti-neutrinos? To answer these questions, neutrino physicists must study how neutrinos and anti-neutrinos mix over time, which means using neutrino interactions to measure their energies and the distances they travel.
If neutrinos and anti-neutrinos do mix differently, it could explain why the universe seems to have so much more matter than antimatter. Answering these questions is difficult because we don’t completely understand how neutrinos interact with matter in the first place. For example, if future experiments see a difference between neutrino and anti-neutrino mixing, it will be hard to determine the reason. On one hand, it could be caused by the neutrino and anti-neutrino actually mixing differently. On the other, it could be a difference between their interactions in the detector, which by definition is made only of matter (no antimatter).
The MINERvA collaboration has recently measured one of the most important interactions for mixing measurements. In this interaction, an anti-neutrino meets a proton, producing a muon and a neutron. This interaction is special because the energy of the anti-neutrino can be estimated simply by measuring the muon energy and direction. However, this isn’t as straightforward as it seems, and that may hamper our ability to infer neutrino energy. For example, because of related measurements from the MiniBooNE experiment of this interaction with neutrinos at lower energies, theorists have hypothesized that interactions sometimes occur in which the neutrino or antineutrino interacts with several particles in the nucleus at once instead of interacting with only one.
The MINERvA collaboration measured this interaction rate versus the square of the four-momentum (which is proportional to the energy) transferred from the anti-neutrino to the neutron. Large contributions from multi-particle interactions, called meson-exchange currents, would change this distribution. So far, MINERvA sees no evidence of such a change, indicating that the anti-neutrino is not interacting with multiple particles. But this is only the beginning: MINERvA has more data and will look at this interaction in many different kinds of nuclei. Stay tuned!