High-Density behavior of symmetry energy and speed of sound in the dense matter within an effective chiral model

TL;DR

This study uses an effective chiral model to explore how mesonic cross couplings influence the symmetry energy and speed of sound in dense matter, with implications for neutron star physics.

Contribution

It introduces a detailed analysis of mesonic cross couplings' effects on symmetry energy and sound speed, identifying the most consistent parameter set with neutron matter constraints.

Findings

The model with J_1=24.6 MeV aligns well with neutron matter constraints.

Speed of sound increases monotonically up to four times saturation density.

Speed of sound approaches the conformal limit in maximum mass stars.

Abstract

With an effective chiral model, we investigate how the mesonic cross couplings $\sigma-\rho$ and $\omega-\rho$ affect the density content of the symmetry energy and its higher-order slope parameters. Earlier mentioned cross-couplings are crucial to controlling the density content of symmetry energy. For this purpose, we did a case study for different values of the symmetry energy $J_1$, defined at density 0.1 fm$^{-3}$ in the range (23.4 - 25.2) for a fixed value of the slope of the symmetry energy $L_0 = 60$ MeV at saturation density and investigate its effect on the higher-order coefficients and their influence on the underlying equation of state. We found that the model with $J_1= 24.6$ MeV is more favorable with the pure neutron matter (PNM) constraints obtained from $\chi$EFT calculations. In addition, we show that all of our models predict a monotonically increasing speed of sound up to four times the saturation density. The speed of sound decreases/saturates above that point and approaches the conformal limit approximately $\sqrt{1/3}~c$ at the center of the maximum mass star.