A tetraneutron is a hypothetical stable cluster of four neutrons. The existence of this cluster of particles is not supported by current models of nuclear forces. There is some empirical evidence suggesting that this particle does exist, based on a 2001 experiment by Francisco-Miguel Marqués and co-workers at the Ganil accelerator in Caen using a novel detection method in observations of the disintegration of beryllium and lithium nuclei. However, subsequent attempts to replicate this observation have failed.
Further work in 2019 suggests potentially observable consequences in neutron star crusts, if the tetraneutron exists.
As detailed at the end of this article, subsequent observations from different ion beam experiments are consistent with short-lived four neutron states with some binding.
As with many particle accelerator experiments, Marques' team fired atomic nuclei at carbon targets and observed the "spray" of particles from the resulting collis