Density-induced dark-baryon conversion in $\Delta -$admixed hypernuclear neutron stars

Abstract

We investigate density-induced conversion of neutrons into a neutral dark baryon $\chi$ in cold, charge-neutral, $\beta$-equilibrated neutron-star matter containing hyperons and all $\Delta (1232)$ quartet. The hadronic sector is modeled within a density-dependent covariant density-functional framework using the DDME2 parametrization. A scalar Higgs portal is included as a possible interaction channel between the visible and dark sectors, although its mean-field contribution is negligible for the couplings adopted here. Unlike fixed dark-matter admixture models or scenarios in which nucleon-to-DM conversion is driven by Higgs exchange, the $\chi$ abundance is determined self-consistently from chemical equilibrium and baryon-number conservation. We find that hyperons and $\Delta$ resonances alter the neutron chemical potential, delay the onset of $\chi$, and suppress its abundance relative to nucleonic matter. This competition induces characteristic changes in the equation of state, particle fractions, sound speed, and adiabatic index. For $m_{\chi }=1250$, $1300$, and $1400$ MeV, the maximum masses of the complete $N+Y+\Delta +\chi$ configurations are $1.806$, $1.899$, and $2.024,M_{\odot }$, respectively, indicating that the massive-pulsar constraint disfavors the lighter dark-baryon benchmarks. The radial profiles further show that for $m_{\chi }=1400$ MeV, $\chi$ is confined to the inner core of the most massive stars, while canonical configurations remain essentially unaffected. Thus, the stellar modifications arise primarily from conversion-induced rearrangement of the equilibrium composition rather than from Higgs-mediated interactions. These results highlight the importance of treating conventional non-nucleonic degrees of freedom and density-generated dark baryons on an equal footing when assessing the astrophysical viability of dark-sector extensions of dense matter.

18 pages, 8 figures. Comments are welcome