‘Impossible’ particle seeks out the edges of physics

In collisions with a particle accelerator in Tokyo, traces of a particle of four neutrons have been found, which according to current theory cannot exist. According to physicists, this is an ideal study object to better understand the forces of atoms.

How does the case really work? Even after almost 90 years of research, physicists are not yet fully aware of how the strong nuclear force that holds charged protons and slightly heavier uncharged neutrons together in atomic nuclei. In the science magazine Nature physicist Meytal Duer (Darmstadt University of Technology) now publishes the discovery of a particle that sharpens our understanding of the powerful nuclear force.

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Members of the research team at the Particle Accelerator in Tokyo. © Thomas Aumann

Standard model

The discovery makes no difference to our daily lives, but it does, if one wants to understand, for example, how the initial phase of the universe proceeded exactly. The so-called standard model of particle physics cannot predict how particles will behave under the conditions prevailing at the time, far beyond the reach of even our largest particle accelerators. Experiments that do not fit the theory perfectly can hopefully give clues as to how the standard model should be modified.

So when Duer and her colleagues decided to take a closer look at the strong nuclear force, they were looking for a particle on the fringes of what the theory can handle. Inside the particle accelerator Factory for radioactive ion beams at the University of Tokyo, therefore, they tried to make a particle that could not actually exist: a tetraneutron, a clump of four uncharged neutrons.

According to our current theory of strong nuclear power, particles of four neutrons are unstable, explains particle physicist Magdalena Kowalska from the particle laboratory CERN. She is not herself involved in the investigation. “This is because the neutrons always arrange themselves in such a way that the particle is under voltage. Sooner or later, therefore, it will fall apart. How fast, it tells you something about how the strong nuclear force behaves and whether it deviates from the existing theory. “

Reaction to make a particle of four neutrons by colliding helium-8 with a hydrogen atom. © Duer et al. / Nature

blow away

To investigate the lifetime of the tetraneutron, Duer started with the helium-8 atom, basically a normal helium atom of two protons and two neutrons, surrounded by four more neutrons. By blasting the inner part of the atomic nucleus away into the particle accelerator, the shell of four neutrons remained.

Based on the energy of the collision products, Dove and her colleagues were able to deduce how long the four neutrons remain together after the collision. The result was an astonishingly short 4 x 10-22 second (ie to 22 decimal places). Even at almost the speed of light, the distance the particle travels at that moment is only a few times the diameter of the original atomic nucleus. In that time, it remains intact, which is why physicists still talk about a particle.

The short life of the tetraneutron is in line with the existing theory: so there is no crack in our understanding of the strong nuclear force. Yet the finding is interesting, according to Kowalska: “According to some theories, no particle with four neutrons would form at all.” Instead, after the collision, the neutrons would go their separate ways. Such alternatives to the standard model are therefore on a slightly looser screw. The researchers hope to be able to refine their measurements further with follow-up research. During their first measurements, the researchers managed to make only 41 of the unstable particles.


By now, not everyone thinks that researchers really have a clue. Particles consisting only of neutrons have been searched in vain for sixty years. In the same edition of Nature Like Doer’s research, independent researchers Lee Sobotka and Maria Piarulli (Washington University) comment on the work. They are convinced that lumps of neutrons are firing, but they are not convinced whether the measured signal really fits a tetra neutron.

It is not surprising that there is doubt about the outcome, says Kowalska. According to the researcher, there are different schools in the research field, each with their own ways of calculating lumps of neutrons and how stable they are. “What one person thinks is a compelling fingerprint of such a particle pushes another aside.”

The study may not provide groundbreaking insight into the powerful nuclear power, but it is a step forward: Dove and her colleagues’ measurements are more accurate than ever before. As a result, other researchers have better data to test their theories on. In recent years, researchers have found more and more cracks in the Standard Model, caused by particles not behaving as expected. Whether the four-neutron clumps contribute to the growing mountain of hints of a better theory of reality, we know only in retrospect.

Sources: Nature, TU Darmstadt

Image: The artist’s impression of a tetra neutron, a particle of four neutrons, which according to current particle physics can hardly exist. © Andrey Shirokov

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