Does a four-neutron resonance exist?
In plain words
Four neutrons alone are not expected to hold together, yet a 2022 experiment saw a short-lived bump in the energy of four outgoing neutrons. Theory disagrees on whether a true resonance (a quasi-bound state that falls apart quickly) can exist.
Precise statement
Duer et al. (Nature 606, 678, 2022) reported a resonance-like structure in the $4n$ system at $E = 2.37 \pm 0.38\,\text{(stat)} \pm 0.44\,\text{(sys)}\,\mathrm{MeV}$ above threshold with width about $1.75\,\mathrm{MeV}$, from quasi-elastic alpha knockout on 8He. Determine whether the $4n$ S-matrix with realistic $\mathrm{NN} + 3\mathrm{N}$ forces has a pole near this energy, or whether the peak is a final-state correlation effect. Answer: pole position, or a proof that no pole lies within $5\,\mathrm{MeV}$ of threshold, with interaction uncertainty.
What would settle it
Complex-scaling and Faddeev-Yakubovsky calculations of the 4n continuum agreeing across methods, plus a second experiment using a different reaction.
Status in the literature
Unverified note
Ab initio calculations disagreed; the most recent lattice EFT study (Wu et al., PRL 136, 142502, 2026) found no resonance but a weak dineutron-dineutron attraction giving a confined 4n energy of 1.7 to 3.3 MeV near the observed peak, favoring a correlation effect without a pole.