January 31, 2023

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The anomaly in the nuclear reactor measurements is not due to a new particle

Zoom in / A schematic 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. It’s far, far lighter than any other particle of mass and only interacts with other matter via the weak force – meaning it hardly interacts with anything. Three types (or flavors) of neutrinos have been identified, and any individual particle has no fixed identity. Alternatively, it can be seen as a quantitative superposition of all three flavors and will oscillate between these identities.

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

The strongest hints of their existence come from strange measurement results in experiments with other flavors of neutrinos. But a new study today rules out sterile neutrinos as an explanation for one such anomaly — even while confirming that the anomalies are real.

Undetectable discovery

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

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Instead, we can detect them through neutrino oscillations. You could set up an experiment that produces a certain type of neutrino at a known rate and then try to detect those neutrinos. If there were sterile neutrinos, some of the neutrinos they produced would oscillate in that identity, and thus not be detected. So you end up measuring fewer neutrinos than you might expect.

This is exactly what used to happen in nuclear reactors. One of the products of radioactive decay (which is driven by the weak force) is neutrinos, so nuclear reactors produce copious amounts of these particles. However, measurements using detectors placed nearby picked up about 6 percent fewer neutrinos than expected. The rapid oscillation in sterile neutrinos could explain this discrepancy.

But these experiences are really hard. Neutrinos rarely interact with detectors so that only a 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, decay quickly turns things into a complex mishmash of new elements, some radioactive and some not. The emitted neutrons can also convert reactor equipment into new isotopes that may be radioactive. Therefore, it is difficult to know exactly how many neutrinos you produce to begin with and the exact fraction of neutrinos you produce that will be recorded by your detector.

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

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