Nucleon Short-Range Correlations and High-Momentum Dynamics: Implications on the Equation of State of Dense Matter

TL;DR

SRCs generate a universal high-momentum tail in the nucleon momentum distribution that is dominated by neutron-proton tensor correlations, quantitatively described by parameters governing the depletion and high-momentum fractions. Incorporating SRC-HMT into both nonrelativistic and relativistic frameworks profoundly alters the kinetic contributions to the equation of state, notably reducing the kinetic symmetry energy and elevating the quartic isospin term, with implications that challenge the conventional isospin parabolic approximation. These microscopic shifts propagate to macroscopic observables: heavy-ion collision signals, neutron-star masses, radii, and tidal deformabilities, as well as sub-threshold particle production and neutrino emissivities, often softening the symmetry energy at sub-saturation densities while modifying high-density behavior. The review also connects SRC-HMT to quasi-deuteron pictures, orbital entanglement entropy, and potential dark-matter couplings, highlighting a cohesive framework that links microscopic nucleon-nucleon correlations to astrophysical phenomenology and experimental observables, and outlining pressing open questions and future directions for theory and experiment.

Abstract

Nucleon short-range correlations (SRCs) and their high-momentum tails (HMTs) encode key short-range dynamics in nuclei and dense matter. This review provides a concise overview of SRC features relevant to the Equation of State (EOS) of isospin-asymmetric nuclear matter. We summarize empirical and theoretical properties of the single-nucleon momentum distribution $n(k)$, emphasizing the role of the neutron--proton tensor force, the dominance of correlated np pairs, and the enhancement of minority-species HMTs. Links to nucleon effective E-masses, quasi-deuteron components, and orbital entanglement are briefly noted. We examine how SRC-induced HMTs modify kinetic and potential contributions to the EOS in both non-relativistic and relativistic frameworks, including the softening of the kinetic symmetry energy and departures from the isospin parabolic approximation of asymmetric nuclear EOS. Sensitivity to high-momentum components and generalizations to arbitrary dimensions are also highlighted. Implications for heavy-ion reactions are summarized, including effects on particle yields, collective flows, deeply sub-threshold particle production and hard photon emission, driven by modified initial nucleon momentum distributions and abundant high relative-momentum np pairs during the reaction. Finally, we outline SRC-HMT consequences for neutron-star matter, covering proton fractions, tidal deformabilities, $Z$-factors, cooling, and the core--crust transition, as well as possible connections to dark-matter interactions in dense environments.

Figures (100)

• Figure 1: (Color Online). Upper: schematic electron energy-loss spectrum, showing elastic scattering, excitation of discrete nuclear levels, excitation of giant resonances, quasi-elastic scattering, pion production and $\Delta(1232)$ formation, $\rm{N}^\ast$ resonance formation and the deep inelastic region. Figure taken from Ref.Frois1987. Lower: sketch of the nucleon momentum distribution $n(k)$ in finite nuclei or in ANM. Majority nucleons dominate the $n(k)$ below the Fermi momentum $k_{\rm F}$, while a relatively larger fraction of minority nucleons reside in the HMT above $k_{\rm F}$ where neutron-proton pairs spatially close, having zero total momentum but large relative momentum are formed by SRCs. Figure taken from Ref.Hen14.
• Figure 2: (Color Online). Back-to-back configuration of a SRC pair in finite nuclei. Figure taken from Ref.Pias23.
• Figure 3: (Color Online). A cartoon on the $n_{\mathbf{k}}$ in finite nuclei, figure taken from Ref.Hen17RMP.
• Figure 4: (Color Online). Upper: the nucleon momentum distribution in $^{2}\rm{H}$, $^4{\rm{He}}$, $^{16}\rm{O}$ and that in infinite SNM; figure taken from Ref.Ben93. Lower: the component of $n(k)$ for $^4\rm{He}$ obtained from the tensor-optimized high-momentum antisymmetrized molecular dynamics (TO-HMAMD). Figures taken from Ref.Lyu20PLB.
• Figure 5: (Color Online). An example on nucleon-nucleon correlation functions for $^{16}\rm{O}$ and $^{40}\rm{Ca}$ for different coupled channels. Figure taken from Ref.Torr18PLB.