Peak of sound velocity, scale symmetry, and nuclear force in baryonic matter

TL;DR

This paper demonstrates that the peak of sound velocity in neutron matter naturally arises at intermediate densities due to scale symmetry effects, providing insights into the equation of state of dense nuclear matter relevant for compact stars.

Contribution

It introduces a novel effective field theory with scale symmetry that explains the emergence of sound velocity peaks without exotic phase transitions.

Findings

Peak of sound velocity occurs at 1-2.5 times nuclear saturation density

The phenomenon is intrinsic to the dilaton compensator approach

Connects scale symmetry to properties of nuclear matter in compact stars

Abstract

The sound velocity in homogeneous matter has fundamental significance as it relates to the stiffness of the equation of state of compact star matter. In this work, we investigate the density evolution of the sound velocity in homogeneous {neutron matter at zero temperature} by using an effective field theory implemented with a conformal compensator -- the nonlinear realization of scale symmetry -- regarded as the source of the lightest scalar meson. We find that the peak of sound velocity emerges naturally in the intermediate density region, $(1-2.5)n_0$, without resorting to any transitions from hadron to exotic configurations or introducing new degrees of freedom. This phenomenon is not found in the Walecka-type models where the sigma meson is included in the linear-type approach, therefore it is an intrinsic character of the dilaton compensator approach through the matching of the QCD trace anomaly; a mechanism has not been found before, and it connects to the character of the lightest scalar meson. In addition, these observations shed light on how the hidden scale symmetry manifests in the nuclear medium from the unitarity limit in dilute matter to the dilaton limit in compact star matter.