S-wave kaon condensation in neutron-star matter within a chiral model framework with dynamical meson masses

Abstract

We investigate s-wave kaon condensation in dense matter and neutron stars within the updated Chiral Mean Field model with an improved meson description (mCMF), which incorporates dynamically generated in-medium meson masses arising from explicit chiral symmetry breaking and vector-meson self-interactions. In contrast to conventional relativistic mean-field descriptions with constant meson masses, the mCMF framework introduces a self-consistent feedback between the meson sector and the dense-matter equations of motion. The kaon dispersion relation is derived from the nonlinear SU(3) Lagrangian, including the Weinberg-Tomozawa interaction and additional baryon-pseudoscalar couplings, and the onset of condensation is determined under conditions of charge neutrality and $β$ equilibrium. Our calculations include the full baryon octet together with electrons and muons at zero temperature. We analyze the impact of hyperons, muons, and kaon condensation on the equation of state, on neutron-star mass--radius relations, and neutron-star thermal evolution, and examine the sensitivity of the onset density and stellar properties to variations in the nucleon--kaon scattering length and to different model vector parameters and vector self-interactions. We find that $K^{-}$ condensation sets in between $n \sim (2-8)\, n_0$ (in units of nuclear saturation density) and leads to a moderate to strong softening (in one case, a slight stiffening of the equation of state), depending on the interplay of kaons and hyperons, while remaining compatible with current $2\,M_\odot$ and small-radius neutron-star observational constraints and producing distinguishable behavior in the neutron-star cooling. This work provides an improved and thermodynamically consistent framework for studying exotic degrees of freedom in neutron-star matter.

Figures (7)

• Figure 1: C4 coupling with nucleons, electrons, and kaons. Left: in-medium kaon mass and electron chemical potential as functions of baryon density. Right: particle number density as a function of baryon density. The dashed line marks the kaon vacuum mass.
• Figure 2: Same as \ref{['fig:nucleons']} but including hyperons and muons.
• Figure 3: Mass-radius diagrams for C4 coupling with nucleons and electrons, without (0)/with (1) hyperons (Y), without (0)/with (1) kaons ($K^-$), and without (0)/with (1) muons ($\mu$). Top left: original kaon couplings $d_1$ and $d_2$. Top right: $9\,d_1,\,9\,d_2$. Bottom: $9\,d_1,9\,_2$ with additional $\omega\rho$ interaction.
• Figure 4: Same as \ref{['fig:hyperons']} but with $9\,d_1,\,9\,d_2$ (top panels) and $9\,d_1,\,9\,d_2$ plus additional $\omega\rho$ interaction (bottom panels).
• Figure 5: Same as the right panels of \ref{['fig:nucleons']}, \ref{['fig:hyperons']}, and \ref{['fig:9times_omegarho']} showing particle number density, as a function of baryon density but for the RC4 parametrization. Top left: with nucleons, electrons, and kaons. Top right: also including hyperons. Bottom left: also including hyperons with $9\,d_1,\,9\,d_2$. Bottom right: also including hyperons with $9\,d_1,\,9\,d_2$ and additional $\omega\rho$ interaction.