Should we search for neutral nuclei? The tetraneutron story

by Dr. Francisco Miguel Marqués (LPC, Caen, France)

Thursday 22 May 2025, 12:30 → 13:30 Europe/Madrid

Salón de Actos (PCUV)

In 1911 Rutherford discovered the atomic nucleus, and in 1919 he demonstrated that the proton, the nucleus of hydrogen, was the carrier of its positive charge and a building block of all the heavier nuclei. Finally, in 1932 the other building block was found, the neutron, which acts as a nuclear glue that holds heavier nuclei together against Coulomb repulsion. A natural question soon emerged: could this glue form neutral nuclei on its own? A quest for these multineutrons started in the 1960s, when techniques to evetually probe them became accessible. In 2002 this quest had been running for 40 years already without success and was fading, but then a first positive signal consistent with a tetraneutron was observed at GANIL.

This result shook the community at several levels. The importance of the potential consequences made it reach the media in many, more or less reliable, ways. Nevertheless, theory was unable to explain the existence of a tetraneutron. Some colleagues, from either theory or experiment, did not believe that our signal was significant enough. And everyone expected a stronger confirmation, or eventually a refutation. However, nothing happened in the next years, at least nothing that fits the streamlined format of standard scientific publications. And yet many things did happen! But most of them had little to do with physics, and those that had did not belong to the linear success tales that we find in physics books and journals.

We will use this first 4n signal to discuss different aspects of basic research. How does/should it work? Why should we search for things that are not supposed to exist? Should we plan our search for the unknown? How should we communicate about experimental failures? Should we allow them? How should we interact with the media? We will also review the present and future of this research. In 2016 something else happened, a new although weak "tetraneutron" signal was reported at RIKEN, and a recent Nature article showed a strong confirmation of a "correlated 4n signal", not a tetraneutron. Is it just a semantic issue? Why does this word matter so much? We will conclude with the next steps in a field that remains still open.

Link to the colloquium