Lattice Calculation of Thermal Properties of Low-Density Neutron Matter with Pionless NN Effective Field Theory

TL;DR

This study uses lattice quantum Monte Carlo methods with pionless effective field theory to analyze the thermal properties and phase transitions of low-density neutron matter, revealing a BCS-BEC crossover.

Contribution

It introduces a consistent EFT-based lattice approach to compute neutron matter properties, including pairing gaps and phase diagram, at low densities.

Findings

Pairing gap is about 30% smaller than BCS predictions.

Critical temperature and pairing temperature scale are determined.

Neutron matter exhibits a BCS-BEC crossover behavior.

Abstract

Thermal properties of low-density neutron matter are investigated by determinantal quantum Monte Carlo lattice calculations on 3+1 dimensional cubic lattices. Nuclear effective field theory (EFT) is applied using the pionless single- and two-parameter neutron-neutron interactions, determined from the $^1S_0$ scattering length and effective range. The determination of the interactions and the calculations of neutron matter are carried out consistently by applying EFT power counting rules. The thermodynamic limit is taken by the method of finite-size scaling, and the continuum limit is examined in the vanishing lattice filling limit. The $^1S_0$ pairing gap at $T \approx 0$ is computed directly from the off-diagonal long-range order of the spin pair-pair correlation function, and is found to be approximately 30% smaller than BCS calculations with the conventional nucleon-nucleon potentials. The critical temperature $T_c$ of the normal-to-superfluid phase transition and the pairing temperature scale $T^\ast$ are determined, and the temperature-density phase diagram is constructed. The physics of low-density neutron matter is clearly identified as being a BCS-Bose-Einstein condensation crossover.