Strongly Interacting Dark Matter admixed Neutron Stars

TL;DR

This paper investigates whether strongly interacting dark matter, described by a lattice-determined $G_2$-QCD EoS, can be admixed in neutron stars without conflicting with current observations. It couples this first-principles dark EoS to a set of ordinary-matter EoS within a two-fluid TOV framework, scanning DM masses in the range of a few hundred MeV to a few GeV and DM fractions up to about 10%. The results show that DM admixture generally reduces the total mass and radius, with lighter DM exerting a stronger influence and creating potential dark-core or dark-halo configurations, while tidal deformability remains broadly compatible with GW constraints for modest DM content. This work demonstrates a UV-complete, lattice-based pathway to test self-interacting DM in compact stars and highlights potential gravitational-wave signatures that could signal DM admixtures in neutron stars.

Abstract

Dark matter may accumulate in neutron stars given its gravitational interaction and abundance. We investigate the modification of neutron star properties and confront them with the observations in the context of strongly-interacting dark matter scenario, specifically for a QCD-like theory with G$_2$ gauge group for which a first-principles equation-of-state from lattice calculations is available. We study the impact of various observational constraints and modeling of the QCD equation of state on the combined neutron stars. The results indicate that dark matter masses of a few hundred MeV to a few GeV are consistent with the latest observed neutron star properties.

Figures (11)

• Figure 1: The equations of state (left) and the speed of sound (right) from Kurkela:2014vha and Wellegehausen:2013cya used in this work. The dark matter candidate mass is chosen to be the mass of the neutron for this plot for comparison. The end of the lines indicate the maximum value for the pressure for the respective equation of state used in this work. The black line indicates $\varepsilon = p$ and $c_s^2=1$ respectively. For the ordinary matter equation of state, the stars denote the results from NChPT while the dots show the endpoints of the piecewise polytropes. For the dark matter equation of state, the points indicate lattice data with polytropes, see appendix \ref{['a:interpolation']}.
• Figure 2: The investigated set of central pressures of ordinary matter ($p_{0,O}$) and dark matter ($p_{0,D}$) for EoS II and the light dark matter equation of state. We will use this combination of ensembles in the main text and show further results in \ref{['a:res']}. Gray points indicate stable solutions with a dark matter fraction >10%. Colored points indicate the dark matter fraction and we indicate solutions with dark matter fractions $<$1% in yellow. The same color scheme is used in the following figures. We show four representative dark matter candidate masses from 0.5 GeV to 4 GeV.
• Figure 3: The mass-radius relation between the total observable mass $M_O+M_D$ in solar masses and the observable ordinary radius $R_O$ in km in the same color-coding and for the same equations of state as in \ref{['plot:pressure']}. The ellipses show results for the mass and radius of various neutron stars, which were determined using different techniques Doroshenko:2022nwpSalmi:2024aumChoudhury:2024xbkLIGO_GW170817.
• Figure 4: Left: The dark matter mass versus the total mass. Right: The dark matter radius versus the ordinary matter radius. The x-axes are chosen such that we can show the neutron star measurements from Doroshenko:2022nwpSalmi:2024aumChoudhury:2024xbkLIGO_GW170817. Note the different y-axes for $R_D$.
• Figure 5: The dimensionless tidal deformability versus the total mass. The yellow and red boxed are gravitational wave observation from LIGO_GW170817 and LIGOScientific:2020aai.