Nuclear chiral density wave in neutron stars?

TL;DR

The paper addresses whether a chiral density wave (CDW) can exist in neutron-star matter. It employs a renormalized nucleon-meson model with a CDW ansatz that rotates the chiral condensate and includes nucleonic vacuum fluctuations. The main finding is that neutron-star conditions disfavor CDW, and realistic two-solar-mass neutron stars arise only in parameter regions without CDW, implying CDW cores are unlikely in typical neutron stars within this framework. The work highlights the sensitivity to vacuum fluctuations and renormalization, and suggests exploring magnetic-field stabilization, pairing effects, alternate models, and holographic approaches for a fuller assessment.

Abstract

Anisotropic phases potentially play a role in the internal composition of neutron stars, the main laboratory for the phase structure of QCD at high baryon densities. We review the study of such a phase, the chiral density wave, within a phenomenological nucleon-meson model, including nucleonic vacuum fluctuations within a renormalization scheme recently developed. Neutron stars in this model and within our approximations either do not contain a chiral density wave core or they are too light to agree with observations.

Figures (4)

• Figure 1: Schematic view of the QCD phase diagram in the space spanned by temperature $T$, baryon chemical potential $\mu_B$, and isospin chemical potential $\mu_I$. It is conceivable that an anisotropic or inhomogeneous phase such as the CDW, is favored in the vicinity of the chiral phase transition (red area). In these proceedings we identify the CDW region at $T=0$ in the parameter space of a nucleon-meson model and test whether realistic neutron stars may have a CDW core.
• Figure 2: Binding energy of pure neutron matter as a function of baryon density $n_B$ normalized to saturation density $n_0$, compared with results from chiral effective field theory given by the green error band Tews:2018kmu.
• Figure 3: Zero-temperature phases in the plane of the neutron chemical potential $\mu_n$ and the nucleon mass at saturation $M_0$, which serves to scan the parameter space of the model. The figure compares the two fits (\ref{['fits']}) (left vs. right), and isospin-symmetric matter (red) with neutron star matter (black). Also, for comparison, the chiral limit (pale curves) is shown. Solid (dashed) lines are phase transitions of first (second) order.
• Figure 4: Upper left panel: Neutron chemical potential in the center of the maximally massive star for both fits, together with the CDW regions from Fig. \ref{['fig:phase']}. Stars with a CDW core exist only in an unstable branch of the mass-radius curve for Fit(ddd), while stable stars with a CDW core are possible for Fit(00d) (see lower panels). The gray segments in the lower panels as well as the dashed segments within the CDW regions in the upper panel are obtained, for comparison, by ignoring the CDW. Upper right panel: Maximum mass of the star in units of the solar mass $M_\odot$ for both fits compared to astrophysical data (orange and pink bands). Asterisks in the upper left panel indicate where the lower boundary of the uncertainty bands of these stars are reached. Only Fit(00d) can reproduce realistic stars. Stars with CDW cores belong to mass-radius curves that do not meet astrophysical constraints.