[Paper Review] What's in the core of massive neutron stars?

Yong-Liang Ma, Mannque Rho|arXiv (Cornell University)|Jun 25, 2020

Pulsars and Gravitational Waves Research22 references4 citations

TL;DR

This paper proposes that the cores of massive neutron stars contain confined quasiparticles of fractional baryon charge, emerging from topological transitions at densities above $3n_0$, due to the spontaneous breaking of scale symmetry and hidden gauge symmetry. These quasiparticles exhibit a pseudo-conformal sound velocity $v_{pcs}^2/c^2 \approx 1/3$, consistent with astrophysical observations and nuclear data, without requiring true quark deconfinement.

ABSTRACT

When hadron-quark continuity is formulated in terms of a topology change at a density higher than twice the nuclear matter densiy $n_0$ the core of massive compact stars can be described in terms of quasiparticles of fractional baryon charges, behaving neither like pure baryons nor deconfined quarks. Hidden symmetries, both local gauge and pseudo-conformal (or broken scale), emerge and give rise to the long-standing quenched $g_A$ in nuclear Gamow-Teller transitions at $\sim n_0$ and to the pseudo-conformal sound velocity $v_{pcs}^2/c^2\approx 1/3$ at $\gsim 3n_0$. These properties are confronted with the recent observations in superallowed Gamow-Teller transitions and in astrophysical observations.

Motivation & Objective

• To explain the equation of state (EOS) of massive neutron stars at densities exceeding $n_0$ using effective field theory beyond standard chiral EFT.
• To resolve the long-standing 'quenched $g_A$' problem in Gamow-Teller transitions by linking it to emergent symmetries at high density.
• To reconcile astrophysical observations of massive neutron stars with nuclear physics data, particularly the near-conformal sound velocity in dense matter.
• To propose a mechanism for hadron-quark continuity via topology change and emergent symmetries, avoiding full quark deconfinement at stellar core densities.

Proposed method

• Formulates a scale-symmetric hidden local symmetry (χmHLS) effective Lagrangian incorporating pions, vector mesons ($\rho,\omega$), and the dilaton ($f_0(500)$) as the Nambu-Goldstone boson of scale symmetry.
• Introduces the 'half-skyrmion' phase as a topological state where baryons fractionalize into quasiparticles with non-integer baryon charge.
• Applies the vector manifestation (VM) mechanism, where vector meson masses vanish at high density, signaling a transition to a topological phase.
• Uses the infrared fixed point (IRFP) of QCD with $N_f=3$ to justify the emergence of pseudo-conformal behavior in the equation of state.
• Derives the pseudo-conformal sound velocity $v_{pcs}^2/c^2 \approx 1/3$ from the interplay of hidden local symmetry and broken scale symmetry.
• Compares predictions with observational constraints using SV interpolation and causality bounds, validating the model against astrophysical data.

Experimental results

Research questions

• RQ1 What is the nature of the core of massive neutron stars at densities above $3n_0$, and how does it differ from pure quark matter?
• RQ2 How can the quenched $g_A$ observed in superallowed Gamow-Teller transitions be explained within a unified framework of nuclear and dense matter physics?
• RQ3 Can emergent symmetries—specifically hidden local gauge and pseudo-conformal scale symmetry—explain the near-conformal sound velocity observed in neutron star interiors?
• RQ4 What is the role of topology and fractional baryon charge in the hadron-quark continuity scenario at high densities?
• RQ5 How does the proposed model reconcile with astrophysical observations of massive neutron stars without requiring full quark deconfinement?

Key findings

• The core of massive neutron stars hosts quasiparticles of fractional baryon charge, arising from a topological transition at densities above $3n_0$, not pure deconfined quarks.
• A pseudo-conformal sound velocity $v_{pcs}^2/c^2 \approx 1/3$ emerges at $\sim 3n_0$, consistent with astrophysical observations and model-independent analyses.
• The quenched $g_A$ in nuclear Gamow-Teller transitions is explained by the same emergent scale symmetry that governs the EOS at high density.
• The model predicts a polytropic index $\gamma < 1.75$ at $\sim 3n_0$, approaching 1 at $n \gtrsim 6n_0$, matching observational constraints.
• The predicted $P/\epsilon$ ratio lies outside the conformality band but parallel to it, indicating pseudo-conformal behavior without full conformal invariance.
• True quark deconfinement is delayed until $n \gtrsim 25n_0$, where the vector manifestation or dilaton fixed point may trigger a Higgs-to-topological phase transition.