Neutron Star Equation of State with Nucleon Short-Range Correlations: A Concise Review and Open Issues

TL;DR

This work reviews how nucleon short-range correlations and the associated high-momentum tail alter the single-nucleon momentum distribution and reshape the dense-matter equation of state. By modifying the kinetic term—notably depressing the kinetic symmetry energy and enhancing the isospin-quartic contribution—and requiring compensatory changes in the potential sector, SRC-HMTs systematically soften the symmetry energy at high density while stiffening it at low density, with model-dependent consequences for neutron-star masses, radii, and tidal responses. The analysis highlights open questions about high-density, isospin-dependent HMT behavior, inner-crust implications, and potential signals that uniquely reveal SRC-HMT effects in neutron stars, calling for integrated efforts across ab initio theory, experiments, and multimessenger astronomy. Overall, SRC-HMTs provide a microscopically grounded link between terrestrial nuclear physics and astrophysical observations, bearing on NS cooling, composition, and structure through the intricate balance of kinetic and potential contributions to the EOS.

Abstract

Nucleon short-range correlations (SRCs) and the associated high-momentum tail (HMT) in its momentum distribution $n(k)$ represent a universal feature of strongly interacting Fermi systems. In nuclear matter, SRCs arise primarily from the spin-isospin dependence of the tensor and short-range components of the nucleon-nucleon interaction, leading to a substantial depletion of its Fermi sea and a characteristic $k^{-4}$ tail populated predominantly by isosinglet neutron-proton pairs. These microscopic structures modify both the kinetic and interaction contributions to the Equation of State (EOS) of dense matter and thereby influence a broad range of neutron-star (NS) properties. This short review provides a streamlined overview of how SRC-induced changes in $n(k)$ reshape the kinetic EOS, including its symmetry energy part and how these effects propagate into macroscopic NS observables, including mass-radius relations, tidal deformabilities, direct Urca thresholds and core-crust transition. We summarize key existing results, highlight current observational constraints relevant for testing SRC-HMT effects, and outline open questions for future theoretical, experimental, and multimessenger studies of dense nucleonic matter.

Figures (13)

• Figure 1: (Color Online). A schematic illustration of the single-nucleon momentum distribution $n_{\mathbf{k}}=n(k)$ in finite nuclei. Below the Fermi momentum $k_{\rm F}$, nucleons predominantly occupy mean-field states, while above the Fermi surface, correlated neutron-proton pairs emerge due to the tensor component of the nuclear force. Figure adapted from Ref.Hen17RMP.
• Figure 2: (Color Online). A summary on NSs observed, these include the three GW events GW170817Abbott2017Abbott2018, GW190425Abbott2020-a and GW190814Abbott2020, the NICER mass-radius joint observations for PSR J0740+6620Riley21, PSR J0030+0451Riley19, PSR J0437-4715Choud24 and PSR J0614-3329Mauv25, the joint X-ray and optical study of redback pulsar PSR J2215+5135Sull24, the black widow pulsar PSR 0952-0607Romani22, the mass of PSR J1614-2230Dem10Arz18 using Shapiro delay and mass of PSR J0348+0432Ant13 via its spectroscopy.
• Figure 3: (Color Online). Left: nucleon momentum distribution $n_{\mathbf{k}}^{J}$ in isospin ANM, showing the SRC-induced HMT above the Fermi surface and corresponding depletion below it. Figure taken from Ref.Cai16c. Right: the back-to-back configuration nature of high-momentum isosinglet neutron-proton pairs, which plays an essential role in shaping the EOS of dense matter and influencing related dynamical processes induced by the incoming projectile (green). Here, "$C$" characterizes the contact strength between an np pair.
• Figure 4: (Color Online). Left: experimental $a_2(A)$ for several typical nuclei, i.e., $^3\rm{He}$, $^4\rm{He}$, $^9\rm{Be}$, $^{12}\rm{C}$, $^{56}\rm{Fe}$ ($^{63}\rm{Cu}$) and $^{197}\rm{Au}$Fom12Wei11Egi06Fra93Sar14. Right: high-momentum nucleon fractions in heavy nuclei and nuclear matter. Figures adopted from Ref.LCCX18.
• Figure 5: (Color Online). Left: the kinetic symmetry energy and the isospin-quartic term including the SRC-induced HMT in comparison with predictions by other theories. Figure taken from Ref.LCCX18. Right: the kinetic symmetry energy using a phenomenological parameterization of Ref.Hen15b; here the parameter $\lambda$ is the high-momentum cutoff.