Binary-boosted Dark Matter

Abstract

We explore the aggregate effect of binary systems on the Milky Way's dark matter (DM) velocity distribution with Monte Carlo simulations. Through gravitational interactions with binaries, transiting DM particles can gain substantial energy. We analyze this mechanism across a range of galactic binaries, and find it to be most effective for double black holes, where ejection speeds can reach $\sim 2000 \ \rm km/s$ while attaining a large ejection rate. We assess the expected binary-boosted DM flux from synthetic populations of black hole binaries in the galaxy, and show direct detection experiments can be sensitive to it. In particular, we demonstrate that large noble liquid detectors such as Lux-Zeplin and PandaX-4T can extend their mass sensitivity down to the sub-GeV scale, and potentially become competitive with other lower-threshold experiments when the full galactic black hole binary population is taken into account. This boosting mechanism, being gravitational in nature, is largely model- and mass-independent.

Figures (15)

• Figure 1: Schematic diagram of the deflection process for a DM particle in the rest frame of the binary's secondary companion (see text for details).
• Figure 2: A sample of observed (or representative hypothetical) binary systems in the Milky Way, classified by semimajor axis. Masses are labeled such that $M_2 \leq M_1$. The ratio of the secondary's escape velocity to the primary's at the orbital distance is related to the deflection probability, while the primary’s escape velocity, shown by the color scale, determines the maximum ejection energy. The circular, equal-mass black hole binaries we focus on in this work are indicated by the label 'BBH', in both cases with total mass $M_1 + M_2 = 28 \, M_\odot$.
• Figure 3: Simulated ejection spectra for a sample of the systems shown in Fig. \ref{['fig:bin_sys_comp_1']}. For ease of comparison, in all cases we assume a background DM density of $\rho_\chi = 0.42 \ \rm GeV \, cm^{-3}$, velocity dispersion $\sigma_\chi = 240 \ \rm km/s$ and Galactic escape velocity cutoff $v_{\rm Gal} = 546 \ \rm km/s$. The latter is indicated in terms of energy by the vertical dashed line.
• Figure 4: Simulated ejection spectra from equal-mass, circular black hole binaries with total mass $M_{1} + M_{2} = 28 \, M_\odot$, for the various periods specified. These assume a background DM density of $\rho_\chi = 0.42 \ \rm GeV \, cm^{-3}$, velocity dispersion $\sigma_\chi = 240 \ \rm km/s$ and Galactic escape velocity cutoff $v_{\rm Gal} = 546 \ \rm km/s$. The vertical dashed line indicates the latter converted to energy.
• Figure 5: Binary-boosted DM velocity distributions from the local and bulge binary black hole populations, assuming all binaries are approximately at rest with respect to the Galactic halo. The Standard Halo Model distribution is shown for comparison, with the vertical dashed line indicating the local Galactic escape velocity. We also display the distribution from a single black hole orbiting Sgr. A$^\star$ to illustrate the potentially dominant contribution from the Milky Way's nuclear star cluster.