Researchers from the FRIB Theory Alliance are probing the limits of what can be considered an atomic nucleus, calculating the possible existence of a tetraneutron, or a four-neutron nucleus.

All known atomic nuclei are made of protons and neutrons that are held together by the nuclear force. But can there exist a nucleus made of one kind of particles, either protons or neutrons? Since protons are charged, they repel one another, so neutrons may offer a better opportunity.

By now we have learned, however, that two neutrons cannot be bound, and adding a third neutron is unlikely to help because nuclei lose stability when neutrons are not paired. But can four neutrons stick together? 

In 2016, an experimental study from the Japanese laboratory RIKEN provided a hint for the existence of an ephemeral four-neutron system, called tetraneutron. The existence of even a very short-lived tetraneutron would profoundly impact our understanding of nuclear matter.

Recent work by National Superconducting Cyclotron Laboratory Research Associate Kevin Fossez et al. [Phys. Rev. Lett. 119, 032501 (2017)], based on key theoretical ingredients, has concluded that the four neutrons cannot stick together for long enough to form a nucleus. So the jury is still out. Hopefully, planned high-statistics experiments will provide an unambiguous experimental answer to the tetraneutron question, which is of great relevance to the problem of neutron-rich matter in the universe. Exploring experimentally what bind nuclei together is one of the areas of research at the future Facility for Rare Isotope Beams.

“The four-neutron system seems to be at the limit of what can be considered a genuine atomic nucleus, so the question of its existence or nonexistence forces us to clarify what is meant by ‘nucleus,’” said Fossez.