Oddities in nuclear reactor measurements not caused by a new particle

Oddities in nuclear reactor measurements not caused by a new particle
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A computer-generated image of a person standing next to an array of equipment placed next to a shielded source of blue radiation.
expand / A diagram of the detector array at STEREO (left) and its location near a nuclear reactor (right).

Loris Scola – CEA

Neutrinos are probably the strangest particles we know. They are much, much lighter than any particle with mass and interact with other matter only through the weak force; which means they hardly interact with anything. Three types (or tat) of neutrinos have been identified, and no single individual particle has a fixed identity. Instead, it can be seen as a quantum superposition of the three flavors and oscillates between these identities.

As if that weren’t enough, a series of strange measurements suggested that there might 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 existence of dark matter, as well as the small masses of other neutrinos, but the whole “impossible to detect” thing makes their existence difficult to address directly.

The strongest clues to their existence come from the strange measurement results in experiments with other types of neutrinos. But today, a new study excludes sterile neutrinos as an explanation for one of these oddities, even while confirming the anomalous results are real.

detecting the undetected

We can detect the presence of particles in two ways: They either interact directly with other substances or decay into one or more interacting particles. This is what makes sterile neutrinos undetectable. They are elementary particles and should not turn into anything. They also interact with other matter only by gravity, and their low mass makes it impossible to detect in this way.

Instead, we can potentially detect them through oscillations of neutrinos. 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 are sterile neutrinos, some of the neutrinos you produce will be released into that identity and thus go undetected. So you measure fewer neutrinos than you expect.

That’s exactly what happened with nuclear reactors. One of the products of radioactive decay (which is driven by the weak force) is a neutrino, so nuclear reactors produce plenty of these particles. However, measurements with nearby detectors picked up about 6 percent fewer neutrinos than expected. A rapid release into sterile neutrinos could explain this discrepancy.

But these experiments are really difficult. Neutrinos interact so rarely with detectors that only a small fraction of what is produced is recorded. And nuclear reactors are incredibly complex environments. Even if you start with a pure sample of a single radioactive isotope, decays quickly turn everything into a complex mixture of new elements, some radioactive and some not. The released 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 what 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 genuine. Physicists tend to take a wait-and-see attitude towards indications that something strange is going on.

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