Physicists claim to spot tetraneutrons.

PHYSICISTS; ATOMIC nucleus; RADIOACTIVE nuclear beams; NEUTRON temperature; ALPHA rays

If confirmed, tetraneutrons would help scientists isolate and probe mysterious neutron-neutron forces and the inner workings of atomic nuclei. When a helium-8 nucleus and a proton collided, the proton knocked out a group of two protons and two neutrons, also called an alpha particle. But theoretical calculations of what happens when four neutrons collide have raised skepticism as to whether a tetra-neutron resonance can exist. [Extracted from the article]