Dineutron

A dineutron is a two-neutron system. There is no vacuum bound state at physical parameters. In the low-energy $s$-wave channel, the spatial state is symmetric, and two neutrons have symmetric isospin; the Pauli exclusion principle therefore requires antisymmetric total spin, the spin-singlet channel. Other orbital angular momentum channels have different exchange constraints. An unbound vacuum pair does not rule out correlated neutron pairs inside a larger atomic nucleus or neutron pairing in dense matter.^[ref]