|The former presence of a cat on the patio can be inferred from where the rain didn’t land. Similarly, sterile neutrinos may be inferred from their effects on normal neutrinos, which themselves are barely visible.|
What does it mean for something to be invisible? If it does not reflect light with the right wavelengths, it is not visible to humans, though it might be detected by a specialized instrument. Neutral particles, such as the neutrons in an atom, do not interact with photons of any wavelength (unless the wavelength is small enough to resolve individual charged quarks within the neutron). Thus, they are invisible to nearly every instrument that uses electromagnetic radiation to see.
However, neutrons are easy to detect in other ways. They interact through the strong and weak nuclear forces, and neutron detectors take advantage of these interactions to “see” them. Neutrinos, on the other hand, are still more invisible, since they have no constituent quarks and interact only through the weak force. Billions of neutrinos pass through every square centimeter per second, but only a handful of these per day are detectable in a room-sized instrument.
Now suppose there were another kind of neutrino that did not interact with the weak force. Physicists would call such a particle a sterile neutrino if it existed. How could it be detected? If something can’t be detected, does it even make sense to talk about it? Could there be a whole world of other particles, filling the same space we do, that can never be detected because they don’t interact with anything that interacts with our eyeballs?
In principle, anything that has mass or energy can be detected because it interacts gravitationally. That is, if there were a sterile neutrino planet right next to the Earth, then it would change the way that satellites orbit: This is our gravitational detector. However, a small mass, such as an individual particle, would deflect orbits so little that it could not be detected in practice.
Although sterile neutrinos would have no effect on ordinary matter, they could be detected through what they do to other neutrinos. Neutrinos of different types mix quantum mechanically. That is, muon neutrinos created by a muon beam can become electron neutrinos and tau neutrinos when they are detected. If there were a fourth, sterile, type of neutrino, then the visible neutrinos would also partly transition to sterile neutrinos in flight and change the fractions of the three visible types of neutrinos in the detector.
In the mid-1990s, an experiment called LSND saw what looked like a sterile neutrino signal, so MiniBooNE, an experiment at Fermilab, studied the effect in more detail. As the MiniBooNE scientists investigated, the story got weirder: the numbers of visible neutrinos didn’t add up, but at different energies than expected. No simple explanation makes sense of the data, but it is possible that a sterile neutrino might. A future experiment, MicroBooNE, will study this phenomenon with higher sensitivity. It would be impressive if the key to new physics is an invisible particle, glimpsed only through its effect on nearly invisible particles!