The isospin quartic term in the kinetic energy of neutron-rich nucleonic matter

TL;DR

This paper reveals a substantial quartic term in the kinetic energy of neutron-rich nucleonic matter, challenging previous assumptions of its negligibility and impacting the understanding of nuclear matter's equation of state.

Contribution

It introduces a novel calculation showing a significant quartic term in the kinetic energy, based on an isospin-dependent momentum distribution constrained by recent experiments and advanced theoretical models.

Findings

The quartic term in kinetic energy is approximately 7.18 MeV.

This quartic term is significantly larger than previously estimated.

Implications for the equation of state of neutron-rich matter are substantial.

Abstract

The energy of a free gas of neutrons and protons is well known to be approximately isospin parabolic with a negligibly small quartic term of only $0.45$ MeV at the saturation density of nuclear matter $\rho_0=0.16/\rm{fm}^3$. Using an isospin-dependent single-nucleon momentum distribution including a high (low) momentum tail (depletion) with its shape parameters constrained by recent high-energy electron scattering and medium-energy nuclear photodisintegration experiments as well as the state-of-the-art calculations of the deuteron wave function and the equation of state of pure neutron matter near the unitary limit within several modern microscopic many-body theories, we show for the first time that the kinetic energy of interacting nucleons in neutron-rich nucleonic matter has a significant quartic term of $7.18\pm2.52\,\rm{MeV}$. Such a large quartic term has significant ramifications in determining the equation of state of neutron-rich nucleonic matter using both terrestrial and astrophysical observables.