Searching for New Physics in Ultradense Environment: a Review on Dark Matter Admixed Neutron Stars

TL;DR

This review analyzes dark matter admixed neutron stars (DANSs) as natural laboratories for DM physics, detailing DM capture, accumulation, and potential outcomes such as DM heating and black-hole formation. It employs a two-fluid Tolman–Oppenheimer–Volkoff framework to study both bosonic and fermionic DM, illustrating how DM cores soften the equation of state and reduce mass-radius, while DM halos can enlarge radii and tidal deformability, depending on DM fraction and interactions. The work highlights DM-induced signatures in gravitational waves, including possible Yukawa-like dark forces altering chirp masses and inducing dipole radiation in mergers, and discusses less conventional channels such as neutron decay into DM with implications for NS stability. It also confronts degeneracies with baryonic EoS, emphasizing that joint analysis of M–R, $\Lambda$–M, $c_s$, and GW/NICER observations across current and future detectors is essential to constrain DM properties and assess the DANS scenario. Overall, NSs emerge as promising astrophysical laboratories to probe DM across wide mass and interaction ranges, informing both particle physics and cosmology.

Abstract

Neutron Stars (NSs), among the densest objects in the Universe, are exceptional laboratories for investigating Dark Matter (DM) properties. Recent theoretical and observational developments have heightened interest in exploring the impact of DM on NS structure, giving rise to the concept of Dark Matter Admixed Neutron Stars (DANSs). This review examines how NSs can accumulate DM over time, potentially altering their fundamental properties. We explore leading models describing DM behavior within NSs, focusing on the effects of both bosonic and fermionic candidates on key features such as mass, radius, and tidal deformability. Additionally, we review how DM can modify the cooling and heating processes, trigger the formation of a black hole, and impact Gravitational Waves (GWs) emissions from binary systems. By synthesizing recent research, this work highlights how DANSs might produce observable signatures, offering new opportunities to probe DM properties through astrophysical phenomena.

Figures (6)

• Figure S1: Summary of astrophysical searches covering the wide range of DM masses probed by various methods. The schematic representation is adapted from Baryakhtar:2022hbu.
• Figure S2: Left panel: Bounds on the (heavy) fermionic DM--nucleon cross-section obtained from the observation of old pulsars in the Milky Way that have not been converted to BHs. Ref. Bramante:2017ulk (from which the plot is taken) also reported constraints from terrestrial direct experiments such as Xenon1T XENON:2019rxpXENON:2019gfnXENON:2019zprXENON:2020gfr and xenon neutrino floor Ruppin:2014bra, as well as potential observations of binary NS mergers accompanied by kilonovae (with localization precision up to 1 kpc within Milky Way-like spiral galaxies). Right panel: Bounds on DM-neutron cross-section obtained with the gravitational collapse condition applied to an old NS with a thermalized core of bosonic DM when a BEC is not formed. The representative NS is assumed to have an average lifetime of $10^{10}$ years and a mean temperature of $10^5$ K, according to Garani et al.'s Garani:2018kkd model, from which the plot is adapted. In the red areas (corresponding to different values of $\rho_\chi$), the accumulated DM triggers the collapse to a BH. In the green areas, DM fails to thermalize with neutrons. The hatched regions indicate where the BH evaporates before it can destroy the NS, thereby weakening the constraints. The Xenon1T limit on DM--neutron cross-sections is represented by the dark blue-shaded region. The updated upper limits from the Lux-Zeplin 2022 LZ:2022lsv and 2024 LZCollaboration:2024lux campaigns are reported in light blue. Note that these experiments explored the WIMP range up to $\sim$$10^4$ GeV (see also Figure \ref{['fig:sigma_DM']} below). Remarkably, in Garani:2018kkd similar analyses have been performed for DM scattering with protons and muons as well as when considering bosonic DM that forms a BEC or fermionic DM.
• Figure S3: Upper limits (68% and 95% CL) on the DM--nucleon cross-section for spin-independent (SI) interactions (top panel) as well as spin-dependent (SD) DM--neutron (bottom left) and DM--proton (bottom right) interactions from the latest public data release of the LUX--ZEPLIN collaboration LZCollaboration:2024lux (from which the plots are adapted). The results are obtained by combining the first 60-live-day exposure (WS2022) with a new 220-live-day exposure campaign (WS2024). Experimental limits derived from WS2022 only LZ:2022lsv, LUX LUX:2016ggv, Xenon1T XENON:2018voc, XENONnT XENON:2023cxc, DEAP3600 DEAP:2019yzn, and PICO--60 PICO:2019vsc are also shown.
• Figure S4: Left: Mass--radius relations for different equations of state of BM (solid lines). The gray-shaded regions are excluded due to GR ($R > 2GM /c^2$), finite pressure ($R > 9GM /(4c^2)$), and causality ($R > 8GM /(3c^2)$) bounds Bucciantini:Lecture_notes, respectively. The rotation of the fastest spinning pulsar, J1748-2446ad Hessels:2006ze, excludes the yellow-shaded area, whereas the red and orange strips correspond to the masses of the two heaviest pulsars: J0740+6620 NANOGrav:2019jurFonseca:2021wxt and J1614-2230 Demorest:2010bx. The figure is taken from Pastor-Marazuela:2022pnp. Right: Mass--radius diagram when DM accumulates within an NS. The plot is adapted from Grippa:2024sfu, where fermionic, interacting DM described by the Lagrangian in Equation \ref{['eq:fermion_lagrangian']} is considered. The black curve corresponds to the BSk22 nuclear EoS, namely, without DM. The other solid lines represent the mass--radius relations with different vector coupling $g_v$ and/or DM fractions $f_\mathrm{DM}$. Configurations highlighted in yellow represent DANSs with a DM halo, otherwise a DM core forms. The blue strip is drawn for GW190814 LIGOScientific:2020zkf; the orange and gray areas correspond to the estimates of the masses of the NSs involved in GW170817 LIGOScientific:2017vwqLIGOScientific:2018cki; the magenta and purple regions come from the 2019 NICER data of PSR J0030+0451 from Miller et al. Miller:2019cac and Riley et al. Riley:2019yda; and the dark and light blue areas represent the 2021 NICER data of PSR J0740+6620 from the same groups Miller:2021qhaRiley:2021pdl.
• Figure S5: $\Lambda$--M relations for a DANS with fermionic DM described by the Lagrangian in Equation \ref{['eq:fermion_lagrangian']}. As in Figure \ref{['fig:mass_radius']}, the black curve is obtained when no DM accumulates, whereas changing $g_v$ and/or $f_\mathrm{DM}$ provides significant modifications. The gray region represents the 90% confidence upper bound from GW170817 LIGOScientific:2018hze, and the light and dark blue regions correspond to the same bounds for the primary and secondary compact objects of GW190425 LIGOScientific:2020aaiYang:2022ees (assuming low-spin priors). Plot is adapted from Grippa:2024sfu.