Neutrino oscillations, discovered 25 years ago, break the Standard Model of particle physics and have been the subject of much investigation.
But even in this new picture, there remain some anomalous measurements suggesting neutrinos could be even stranger.
Neutrinos are very strange particles indeed. Neutrinos have a property known as ‘flavour’, which dictates which charged particle they pair with – the humble electron or its lesser-known and heavier cousins, the muon and tau. But oddly, as neutrinos travel, they can change from one flavour to another!
This actually isn’t quite as impossible as it sounds – quarks, the particles that form protons and neutrons, have a similar property, and this comes from the fact that in the quantum realm, things can be in a mixture (or ‘superposition’) of different states. For both quarks and neutrinos, there are three masses and three flavours, but they don’t line up. The lowest mass neutrino is a combination of electron, muon, and tau flavour. It may sound far-fetched, but the maths adds up and matches our data.
The thing about neutrinos
The big problem for neutrinos is that the Standard Model has no way of producing neutrino masses, so there shouldn’t be any mass states to mix up. Putting that problem to one side, there are three neutrino flavours, so there should be three masses.
The rate at which the flavours change depends on three things: the distance the neutrino travels, its energy, and the difference in neutrino masses. By picking the energy and distance, an experiment will be sensitive to a particular mass difference. Three neutrino masses mean two mass differences should be enough to describe all measurements, so we should only see oscillations at two distance/energy pairs, in theory.
There are two mass differences established: the ‘solar’ mass difference of around 10-5 electronvolts (eV) and the ‘atmospheric’ mass difference of around 10-3 eV.
However, a few experiments have now seen indications of oscillations at mass differences of around one electronvolt. Where can this third mass difference come from? Is it an experimental mistake, another neutrino, or some other kind of new physics?
Other experiments have searched for these ‘extra’ neutrino oscillations, and many have failed to see any effect, but no experiment is completely foolproof, and it’s still possible that there is another neutrino making these oscillations happen.
The Fermilab experiment
The Fermilab Short Baseline Neutrino (SBN) programme pairs new detector technology with a well-understood beam. Importantly, there are three detectors in the beam, placed at three very different distances, with the closest detector at 110m, the furthest at 600m, and another in the middle at around 450m. If neutrinos are changing as a function of distance, as the theory suggests, the three detectors will see three different things.
The first of these detectors to operate is the ‘middle’ one, known as ‘MicroBooNE’, and has already searched for evidence of these oscillations by itself. The early data shows no real evidence of an anomaly, but the experimental collaboration is currently working on processing its full dataset (twice as much as the first measurement used) and improving the analyses to enhance the sensitivity of the data.
However, when the new detectors come online, the sensitivity should be able to easily cover all possible explanations for the anomalous results. This isn’t because the new detectors are better; it’s just because there’ll be multiple detectors. The reason having multiple detectors is so important comes down to uncertainties.
It’s hard to say exactly how many neutrinos are produced in the beam and what energies they have, exactly. It’s also hard to predict how neutrinos interact in detectors and exactly how detectors respond to those neutrinos.
So, when we only have a single detector, we’re comparing measured data to an uncertain prediction. The ‘smoking gun’ signature of these oscillations is how they change with distance, and we can see a change by measuring the same beam multiple times.
If your detectors are identical (or close to it), the beam is the same, and they are interacting with the same material, then all these uncertainties have the same impact on all three detectors, but they won’t lead to differences between what each one sees.
This same multi-detector strategy has been used extensively by ‘standard’ oscillation experiments for many years. We’re now adopting it to see if we can spot any other oscillations. This will be the most stringent test yet of this potentially anomalous behaviour. Whatever we find, we should settle a 20-year-old question!
Please note, this article will also appear in the seventeenth edition of our quarterly publication.
