In a paper recently published in Physics Letters B, we analyzed data from a 2018 experiment to search for the possibility of two neutrons forming a short-lived bound state. The experiment was conducted to study how protons move in helium-3 (two protons and one neutron) and tritium (two neutrons and one proton) nuclei. We realized that the data could reveal the presence of a bound di-neutron. Our analysis showed no evidence of any such state.
When a proton is knocked out of helium-3, one proton and one neutron are left behind. These can fly apart, or they can stay bound as a deuterium nucleus, called a deuteron. By contrast, when a proton is knocked out of tritium, two neutrons are left behind. No one has ever observed a “di-neutron” state, and so it is believed that the two neutrons always fly apart. However, if in some small fraction of collisions they could bind, then the signature in the data would look exactly like the deuteron signature in helium-3. The helium-3 data provide a sort of “template” that allowed us to search of a di-neutron without having to model exactly how it would appear in our detectors.
Looking at the data, we saw no evidence for a bound di-neutron state. This allowed us to place exclusion limits on possible di-neutron formation in tritium.