Is a Trineutron Resonance Lower in Energy than a Tetraneutron Resonance?

TL;DR

This study uses quantum Monte Carlo methods with chiral interactions to estimate neutron resonances, suggesting that the three-neutron resonance is lower in energy than the four-neutron resonance, which has implications for nuclear physics and ultracold gases.

Contribution

It introduces a novel approach to estimate neutron resonances by extrapolating from external potential calculations, providing new insights into the energy ordering of three- and four-neutron states.

Findings

The extrapolated trineutron resonance is lower than the tetraneutron resonance.

The approach reproduces odd-even staggering in helium isotopes.

Results are analogous to phenomena in ultracold Fermi gases.

Abstract

We present quantum Monte Carlo calculations of few-neutron systems confined in external potentials based on local chiral interactions at next-to-next-to-leading order in chiral effective field theory. The energy and radial densities for these systems are calculated in different external Woods-Saxon potentials. We assume that their extrapolation to zero external-potential depth provides a quantitative estimate of three- and four-neutron resonances. The validity of this assumption is demonstrated by benchmarking with an exact diagonalization in the two-body case. We find that the extrapolated trineutron resonance, as well as the energy for shallow well depths, is lower than the tetraneutron resonance energy. This suggests that a three-neutron resonance exists below a four-neutron resonance in nature and is potentially measurable. To confirm that the relative ordering of three- and four-neutron resonances is not an artifact of the external confinement, we test that the odd-even staggering in the helium isotopic chain is reproduced within this approach. Finally, we discuss similarities between our results and ultracold Fermi gases.