Peculiarities in nuclear reactor measurements not due to a new particle
Neutrinos are probably the strangest particles we know of. They are much, much lighter than any other particle with mass and only interact with other matter via the weak force – meaning they almost never interact with anything. Three types (or flavors) of neutrinos have been identified, and each individual particle has no fixed identity. Instead, it can be viewed as a quantum superposition of all three flavors and will oscillate between these identities.
As if all that wasn’t enough, a series of strange measurements have suggested 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 possibly explain the small masses of the other neutrinos, as well as the existence of dark matter, but the whole “impossible to detect” thing makes it hard to address their existence directly.
The strongest clues to their presence come from strange measurements in experiments with other flavors of neutrinos. But a new study today rules out sterile neutrinos as an explanation for any of these oddities — even while confirming that the anomalous results are real.
Discovering the undetectable
We can detect the existence of particles in two ways: they interact directly with other matter, or they decay into one or more particles that do. That’s what makes sterile neutrinos undetectable. They are fundamental particles and must not decay into anything. They also only interact with other matter via gravity, and their low mass makes detection via this route impossible.
Instead, we may be able to detect them through the oscillations of neutrinos. You can set up an experiment that produces a specific type of neutrino at a known rate and then try to detect those neutrinos. If there are sterile neutrinos, some of the neutrinos produced by you will oscillate in that identity and thus go unnoticed. So you end up measuring fewer neutrinos than you would expect.
That is exactly what has happened with nuclear reactors. One of the products of a radioactive decay (which is driven by the weak force) is a neutrino, so nuclear reactors produce large amounts of these particles. However, measurements with nearby detectors yielded about 6 percent fewer neutrinos than expected. A fast oscillation in sterile neutrinos could explain that discrepancy.
But these experiments are really hard. Neutrinos so rarely interact with detectors that only a small fraction of those produced are recorded. And nuclear reactors are incredibly complex environments. Even if you start with a pure sample of a single radioactive isotope, the decay quickly turns into a complicated mix of new elements, some radioactive, some not. The released neutrons can also convert the reactor equipment into new isotopes that may be radioactive. So it’s hard to know exactly how many neutrinos you’re producing to begin with and the exact fraction of the ones you’re producing that will be registered by your detector.
For all these reasons, it is difficult to be sure that deviations in neutrino measurements are real. Physicists tend to take a wait-and-see approach to clues that something fishy is going on.
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