Repulsive vector interaction as a trigger for the non-conformal peak in $V_s^2$

TL;DR

This paper demonstrates that a density-dependent repulsive vector interaction in the NJL model can produce a non-monotonic speed of sound peak above 1/3, aligning with QCD trace anomaly positivity and affecting quark star properties.

Contribution

It introduces a density-dependent vector coupling in the NJL model to generate a non-conformal peak in the speed of sound, reconciling it with QCD constraints.

Findings

Non-monotonic $V_s^2$ peak above 1/3 achieved with density-dependent $G_V$

Non-conformal peak compatible with positive QCD trace anomaly

Implications for mass-radius relations of quark stars

Abstract

Considering the NJL model with a repulsive vector channel, parametrized by $G_V$, I show that one may generate a non-monotonic behavior for the speed of sound which peaks at $V_s^2 > 1/3$. This can be achieved by assuming $G_V$ to be density dependent so that the resulting EoS is stiff/repulsive at low densities and soft/non-repulsive at high densities. The interpolation between the two regimes happens through a cross-over which takes place after the first order chiral transition sets in. The model explicitly shows that a non-conformal peak in $V_s^2$ is not in tension with the QCD trace anomaly being positive at all densities, supporting recent claims in this direction. A brief discussion on how the running coupling may affect the mass-radius relation is carried out in the context of simple non-strange quark stars.