Surface and curvature tensions of cold dense quark matter: a term-by-term analysis within the Nambu-Jona-Lasinio model

Abstract

In this paper, we conduct a thorough investigation of the surface and curvature tensions, $σ$ and $γ$, of three-flavor cold quark matter using the Nambu-Jona-Lasinio (NJL) model with vector interactions. Our approach ensures both local and global electric charge neutrality, as well as chemical equilibrium under weak interactions. By employing the multiple reflection expansion formalism to account for finite size effects, we explore the impact of specific input parameters, particularly the vector coupling constant ratio $η_V$, the radius $R$ of quark matter droplets, as well as charge-per-baryon ratio $ξ$ of the finite size configurations. We focus on the role of the contributions of each term of the NJL Lagrangian to the surface and curvature tensions in the mean field approximation. We find that the total surface tension exhibits two different density regimes: it remains roughly constant at around $100 \, \mathrm{MeV \, fm^{-2}}$ up to approximately $2-4$ times the nuclear saturation density, and beyond this point, it becomes a steeply increasing function of $n_B$. The total surface and curvature tensions are relatively insensitive to variations in $R$ but are affected by changes in $ξ$ and $η_V$. We observe that the largest contribution to $σ$ and $γ$ comes from the regularized divergent term, making these quantities significantly higher than those obtained within the MIT bag model.