In a recent PRL article Artemis Spyrou and collaborators have reported on the first observation of an exotic form of nucleons, the long-sought dineutron.

Dineutron was predicted theoretically as a short-lived bound state of two neutrons emitted from a nucleus. The formation is possible due to the strong nuclear force that may hold the two neutrons close by for some time.

The dineutron is not easy to get. You have to find it as a decay product in some rare decay of an exotic, almost unbound nucleus under at least one special requirement: the nucleus emitting the dineutron should show a larger probability of decaying by the emission of the dineutron that emitting two successive neutrons independently. Earlier searches of the dineutron involved exotic isotopes such as hydrogen-5 or helium-10 as the appropriate candidates for such a rare decay.

In helium-8, the outmost, almost unbound neutrons are supposed to be in the form of loosely bound dineutron in the He-6 core. However, no emissions had been observed from that nuclear system.

Certainly, the research case needs to be looked further to provide fundamental information on the properties of the dineutron. As new facilities employing radioactive beams are built and upgraded, I think there is a good chance I will write more on the subject in the future. Till then, you may read the original article here: 10.1103/PhysRevLett.108.102501