A thorough search for the sterile neutrino has turned up nothing.

The Standard Model of particle physics is very clear about neutrinos: the subatomic, elementary particles come in three ‘flavors’. You have the electron neutrino, the muon neutrino and the tau neutrino. But when researchers started working with particle accelerators a few decades ago, they came across a surprise. “Aberrations were registered,” Professor Camillo Mariani tells Scientias.nl. And those deviations hinted at the existence of a fourth ‘flavor’. “The fourth neutrino was also referred to as the sterile neutrino. Why sterile? Because it doesn’t interact with the other neutrinos.”


For years, this hypothetical elementary particle seemed to provide a good explanation for what researchers had seen happening in those particle accelerators. But there was no evidence that the particle actually existed. Reason enough for Mariani and colleagues to actively look for it.

Studying subatomic particles is not easy. For example, researchers cannot directly observe neutrinos. What they can see in detectors are the particles that are formed when neutrinos in such a detector hit an atom.

The researchers used a time projection chamber filled with liquid argon for this. “This time projection room allows us to get a very detailed picture of the interaction between particles,” explains Mariani. “You can compare it a bit with the resolution of a 4k Ultra HD TV compared to the resolution of the thick picture tubes I grew up with. Higher resolution means more vivid and detailed images. And if you want to understand how things work, it’s always good to have more details at your disposal.” But to the disappointment of the researchers, there was no trace of the sterile neutrino in the fine details. “It’s shocking that we haven’t found a new particle,” Mariani says.

Deviations are again inexplicable

The results are in line with the Standard Model, which, as mentioned, predicts the existence of only three neutrinos. But the findings do not alter the fact that researchers have seen something in previous experiments that the Standard Model cannot explain. “There must have been something that led to abnormalities being recorded in previous experiments,” Mariani says. “We have to understand what that was. It was not a sterile neutrino, but it could have been dark matter or something else.”

That’s how science works

And so Mariani and colleagues’ experiments actually raise more questions than answers. “This is how science works,” Mariani soberly notes. “You’re investigating something and you have to prove that you’re right or wrong. You can’t choose what you discover, you observe and report your results objectively. It would have been nice if we had discovered a new particle and then probably won the Nobel Prize for it. But that was not the case and although we are a bit disappointed about that, we have learned something in the meantime. In the sense that we have disproved the previous hypothesis. We must now continue to work to explain the discrepancies.”

Other hypotheses

There are already alternative hypotheses. For example, we think of dark matter. Another possibility is that the also hypothetical Z-prime boson plays a role. “The new data takes us away from the likely explanations and points to something much more complex and interesting, which is very exciting,” said study researcher Sam Zeller.

sterile neutrino

In addition, you should not be surprised if in a few years it turns out that the sterile neutrino has something to do with it. Because, Mariani explains, physicists have a certain view of the sterile neutrino. And that’s what they’re looking for now. Without success. “It could mean that nature doesn’t have a sterile neutrino. Or that the sterile neutrino does not show up at the energies and distances that we work with in our detector (…). For example, it is possible that the sterile neutrino has a much higher energy than thought.” In that scenario, the sterile neutrino may go undetected now, but just pop up during future searches.

The importance of neutrinos

Scientists are determined to unravel the mysteries surrounding neutrinos. “We have some big, unanswered questions in physics that we’re currently trying to solve through experiments,” said Professor Bonnie Fleming. “And neutrinos can tell us where to find the answers. I think if you want to know how the universe works, you have to understand neutrinos.”

Very close

And whoever thinks it’s all quite abstract and a far-from-my-bed show is wrong. One of the most pressing questions physicists are asking, and which neutrinos seem to have something to do with, is why we—and everything we see around us—is there. That question leads us to the beginning: the big bang. “As much matter as antimatter was created, Mariani explains. And when antimatter particles and matter particles collide, they destroy each other. Since there were equal numbers of both, you would expect there to be nothing today. “But due to some process, matter has come to dominate. And so we are here. We need to understand why that was.” And neutrinos may help explain the dominance of matter.

It is just one example of a large demand that can be reduced to very small particles. And there are many more, Mariani emphasizes. “There are countless phenomena in our environment that we would like to know why they are there. Look around you and see how the basic laws of nature are the driving force behind everything. And we are eager to reveal those laws of nature.” This is not without a struggle, as is once again apparent from the latest study by Mariani and colleagues. But surprises and setbacks also take science further. “It’s a little sad that we didn’t manage to discover a new particle, but we’ve increased our knowledge about nature. And that is very important.”