Neutrinos are probably the strangest particles we know about. They’re far, far lighter than any other particle with mass and only interact with other matter via the weak force—which means they barely ever interact with anything. Three types (or flavors) of neutrinos have been identified, and any individual particle doesn’t have a fixed identity. Instead, it can be viewed as a quantum superposition of all three flavors and will oscillate among these identities.
As if all that weren’t enough, a set of strange measurements has suggested that there could be a fourth type of neutrino that doesn’t even interact via the weak force, making it impossible to detect. These “sterile neutrinos” could potentially explain the tiny masses of the other neutrinos, as well as the existence of dark matter, but the whole “impossible to detect” thing makes it difficult to address their existence directly.
The strongest hints of their presence come from odd measurement results in experiments with other flavors of neutrinos. But a new study today rules out sterile neutrinos as an explanation for one of these oddities—even while confirming that the anomalous results are real.