Stability of the dineutron bound state at intermediate pion masses

Ascertain whether the two-neutron (dineutron) state in Quantum Chromodynamics is stable or unbound for pion masses in the range 135–300 MeV, for example via lattice QCD calculations, to clarify the conditions under which dark-sector dineutron analogues might form during dark Big Bang nucleosynthesis.

Background

To prevent atomic dark matter and satisfy self-interaction constraints, the scenario requires dark matter to be predominantly dark neutrons rather than dark protons or dark nuclei. A potential loophole arises if dineutron bound states exist: even an initially all-neutron plasma could form dark nuclei without Coulomb repulsion, threatening cosmological consistency. In the Standard Model, dineutron is not bound; however, lattice QCD indicates stability for m_π ≥ 300 MeV, leaving uncertainty in the intermediate regime.

The authors therefore impose a conservative constraint on dark-sector parameters to avoid dineutron binding, but note that the stability of the dineutron for 135–300 MeV pion masses is not established. Resolving this would sharpen the allowed dark-sector parameter space and improve the robustness of the model’s cosmological predictions.