Dark neutron stars from a heavy dark sector

TL;DR

This work presents a sequestered two-sector MSSM-like framework in which the dark sector has a much higher SUSY-breaking scale $M_{ ext{SUSY}}' \gg M_{ ext{SUSY}}$, naturally yielding comparable baryon densities via the relation $\rho_B'/\rho_B = O(1) (M_{ ext{SUSY}}'/M_{ ext{SUSY}})^{\delta_n-\epsilon}$. The authors argue that, with a dark quark hierarchy $m_{u'} \gtrsim m_{d'}$, the dark neutron is the lightest stable baryon while the dark proton is long-lived enough to permit a temporary dissipative phase and cooling, enabling fragmentation of ionized sub-halos into dark stars and ultimately dark neutron stars or black holes. For representative parameters ($M_{ ext{SUSY}}' \sim 10^8$ GeV, $m_{e'} \sim 10$ MeV, $m_{n'} \sim 1$ TeV), dark neutron stars with masses around $M_{NS}' \sim 10^{-6} M_\odot$ and radii of order centimeters can form, potentially producing detectable signals via gravitational microlensing and low-frequency radiation through photon-dark photon kinetic mixing with $\epsilon \lesssim 10^{-8}$. The framework offers a testable link between the dark matter abundance and compact objects, with observational probes ranging from microlensing to radio astronomy, while remaining consistent with Big Bang Nucleosynthesis bounds and avoiding premature dark-sector thermalization. Future work should include detailed simulations of dark star formation and a more thorough treatment of reheating to refine the viable parameter space.

Abstract

We study the formation and properties of dark neutron stars in a scenario where dark matter is made up of (heavy) dark baryons in a sequestered copy of the MSSM. This scenario naturally explains the coincidence of baryonic and dark matter abundances without the need for tuning particle masses. In particular, the supersymmetry breaking scales in the visible and dark sectors may differ by up to 10-11 orders of magnitude. We argue that dark neutrons should be the lightest dark baryons, but that dark protons may be cosmologically long lived. This allows a small fraction of dark matter to remain ionized until the first halos start to form, providing cooling mechanisms that foster the gravitational collapse and fragmentation of sub-halo structures, ultimately resulting in dark neutron star and black hole formation. For a wide range of model parameters, we find dark neutron stars with generally smaller mass and radius than ordinary visible sector neutron stars. We also discuss their potential detectability, particularly through gravitational microlensing and dark magnetic dipole radiation at radio frequencies through photon-dark photon kinetic mixing.

Figures (4)

• Figure 1: Region of parameter space with at least percent-level residual ionization of the dark sector, $\chi_{e'} > 0.01$. Several values of the temperature ratio of the two sectors are shown in blue shaded regions (with larger $T'$ excluded; see surrounding text). Of interest for dark star formation is the overlap with the green region, for which masses the dark proton is long lived (here we have set $\mathcal{F} \sim \mathcal{O}(1)$).
• Figure 2: Evolution (black line) of dark proton/electron number density and temperature within a dark matter sub-halo. Jeans mass contours (gray lines) are labeled by $\log(M_J/M_\odot)$, with the blue contour denoting the Chandrasekhar mass, $M_J = M_C$. The range of $T'$ is restricted to the non-relativistic regime, $T' < m_{e'}$. In the overlapping green regions, cooling by dark electron-electron bremsstrahlung and dark electron-proton bremsstrahlung, respectively, are efficient ($t_\text{ff} \simeq t_\text{cool}$); in the gray region the cooling time exceeds $H_0^{-1}$. Fragmentation occurs as the trajectory moves towards smaller Jeans mass values. Cooling and fragmentation cannot occur in the red regions, where emitted dark photons cannot escape the star. Fragmentation is expected to stop around $M_J \lesssim M_C$ where we draw the trajectory as a dashed line.
• Figure 3: Kinetic mixing of the two photons allows a direct coupling between the visible and dark sectors e.g. via two-particle scattering of visible and dark fermions. We constrain the kinetic mixing parameter $\epsilon$ to avoid such processes thermalizing the dark sector, which would spoil the dark cosmological evolution discussed in Sec. \ref{['sec:evolution']}.
• Figure 4: Dark electromagnetic coupling $\alpha^{\prime}$ computed at $m_{e'}$; see Eq. (\ref{['alphaEMIRfull']}). The low energy coupling $\alpha'$ of the dark sector is slightly larger than in the visible sector. For example, with $M_\text{\tiny{SUSY}}'\sim 10^{12}$ GeV, dark electromagnetism is somewhat stronger, $\alpha'=1/100>1/137$.