This work presents further analysis of the three- and four-neutron systems in the low energy regime using adiabatic hyperspherical methods. In our previous article [M. D. Higgins et al., Phys. Rev. Lett. 125, 052501 (2020)], the low-energy behavior of these neutron systems was treated in the adiabatic approximation, neglecting the off-diagonal nonadiabatic couplings. A thorough analysis of the density of states through a multichannel treatment of the three-and four-neutron scattering near the scattering continuum threshold is performed, showing no evidence of a $4n$ resonance at low energy. A detailed analysis of the long-range behavior of the lowest few adiabatic hyperspherical potentials shows there is an attractive ${\rho }^{-3}$ universal behavior which dominates in the low-energy regime of the multichannel scattering. This long-range behavior leads to a divergent behavior of the density of state for $E\to 0$ that could account for the low-energy signal observed in the 2016 experiment by Kisamori et al. [Phys. Rev. Lett. 116, 052501 (2016)].

• Received 23 November 2020
• Accepted 12 February 2021

DOI:https://doi.org/10.1103/PhysRevC.103.024004