Oddities in nuclear reactor measurements not due to a new particle

Enlarge / A diagram of the detector array in STEREO (left) and its location near a nuclear reactor (right).

Loris Scola – CEA

Neutrinos are perhaps the strangest particles we know. They are much, much lighter than any other particle with mass and interact with other matter only through the weak force – meaning they almost never interact with anything. Three types (or flavors) of neutrino have been identified, and no individual particle has a fixed identity. Instead, it can be seen as a quantum overlap of all three flavors and will oscillate between these identities.

As if all that weren’t enough, a series of strange measurements have suggested that there may be a fourth type of neutrino that doesn’t even interact via the weak force, making detection impossible. These “sterile neutrinos” could potentially explain the small masses of other neutrinos, as well as the existence of dark matter, but the whole “impossible to detect” thing makes it difficult to directly address their existence.

Stronger hints of their presence come from strange results of measurements 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 as it confirms that the anomalous results are real.

Knowing the undiscoverable

We can detect the existence of particles in two ways: They either interact with other matter directly, or they decay into one or more particles that they do. This is what makes sterile neutrinos undetectable. They are fundamental particles and should not decay into anything. They also interact with other matter only through gravity, and their low masses make detection through this route an impossibility.

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Instead, we could potentially detect them via neutrino oscillations. You can set up an experiment that produces a specific type of neutrino at a known speed and then try to detect those neutrinos. If there are sterile neutrinos, some of the neutrinos you produced will oscillate in that identity and, thus, remain undetected. So you end up measuring fewer neutrinos than you expect.

This is exactly what has happened in nuclear reactors. One of the products of radioactive decay (which is driven by the weak force) is a neutrino, so nuclear reactors produce copious amounts of these particles. However, measurements with detectors located nearby picked up about 6 percent fewer neutrinos than expected. A rapid oscillation in sterile neutrinos may explain this discrepancy.

But these experiments are really difficult. Neutrinos interact with the detectors so rarely 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 decays quickly turn things into a complicated mixture of new elements, some radioactive, some not. The released neutrons can also convert 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 those you produce that will be recorded by your detector.

For all these reasons, it is difficult to be sure that any anomalies in neutrino measurements are real. Physicists tend to take a wait-and-see attitude toward indications that something strange is going on.