To which densities is spin-polarized neutron matter a weakly interacting Fermi gas?

TL;DR

This paper investigates spin-polarized neutron matter using chiral effective field theory, finding it behaves similarly to a weakly interacting Fermi gas up to saturation density, with potential implications for neutron star physics.

Contribution

It provides the first detailed next-to-next-to-next-to-leading order analysis of spin-polarized neutron matter including multi-neutron interactions, highlighting its near non-interacting behavior.

Findings

Neutron matter energy is within 10% of a non-interacting system up to saturation density.

Interactions have minimal impact, making the system similar to a unitary gas.

Results are testable with lattice QCD and vary with pion mass.

Abstract

We study the properties of spin-polarized neutron matter at next-to-next-to-next-to-leading order in chiral effective field theory, including two-, three-, and four-neutron interactions. The energy of spin-polarized neutrons is remarkably close to a non-interacting system at least up to saturation density, where interaction effects provide less than 10% corrections. This shows that the physics of neutron matter is similar to a unitary gas well beyond the scattering-length regime. Implications for energy-density functionals and for a possible ferromagnetic transition in neutron stars are discussed. Our predictions can be tested with lattice QCD, and we present results for varying pion mass.