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.

• The paper establishes gravitational scattering in compact binaries as a model-independent dark-matter acceleration mechanism, with double black holes producing particles up to roughly 2,200 km/s and Sgr A* systems up to 8,300 km/s.
• The authors use large Monte Carlo three-body simulations and Galactic binary populations to show that short-period double black-hole binaries dominate the high-energy ejecta, while nuclear-star-cluster sources may provide the largest overall flux.
• The boosted population could extend LZ and PandaX-4T sensitivity to approximately 0.6–0.9 GeV dark matter and raise xenon inelastic-scattering thresholds to about 800–900 keV, with MeV-scale reach possible near the Galactic Center.

Overview

This paper by Acevedo and Ritz (2603.08781) identifies a purely gravitational mechanism for accelerating a sub-component of the Milky Way's dark matter (DM) to speeds well above the conventional halo cutoff. Dark matter particles transiting binary systems undergo close gravitational deflections that can transfer orbital kinetic energy to the particle, continuously generating an unbound, high-velocity population. Because the mechanism relies only on Newtonian gravity, it is largely independent of both the DM mass and its non-gravitational couplings to the Standard Model — a property that distinguishes it from cosmic-ray, solar-reflection, or supernova boosting channels, which typically require specific interaction assumptions or suffer from flux suppression.

The central quantitative results are that short-period double black hole binaries can eject particles at up to $\sim 2000$–$2200\ \rm km/s$, while black holes bound to Sagittarius A$^\star$ can reach $\sim 8300\ \rm km/s$. The authors show that this boosted flux extends the mass sensitivity of LZ and PandaX-4T down to roughly $0.5$–$1\ \rm GeV$ for spin-independent scattering, and substantially widens the kinematically accessible mass splittings for inelastic DM at TeV masses.

Analytic picture of gravitational ejection

In the restricted three-body limit ($m_\chi \ll M_2 \ll M_1$), the energy gain is understood as a Galilean-frame effect: elastic hyperbolic deflection in the rest frame of the secondary companion, followed by a boost back to the primary's frame. The energy change is

$$\Delta \varepsilon = u_{M_2} v_i \left(\cos\beta - \cos(\beta+\chi)\right),$$

where $\beta$ parameterizes the approach direction relative to the companion's motion and $\chi$ is the scattering angle set by $2200\ \rm km/s$0. The maximum gain, $2200\ \rm km/s$1, occurs for head-on reversal ($2200\ \rm km/s$2, $2200\ \rm km/s$3), implying that systems with deep potential wells — large orbital velocities and large infall speeds — maximize the ejecta energy.

The authors are explicit about the limits of this estimate: for compact equal-mass black hole binaries the spheres of influence overlap, producing multi-body interactions whose ejections can exceed the single-deflection estimate by order-unity factors. The analytic treatment also provides no probability distribution over ejecta energies, motivating the numerical program that follows.

Monte Carlo simulation

The simulation evolves ensembles of $2200\ \rm km/s$4 to $2200\ \rm km/s$5 three-body realizations with a Velocity Verlet integrator. Binaries are initialized on exact Keplerian orbits; DM particles are injected on a bounding sphere of $2200\ \rm km/s$6 semimajor axis with truncated Maxwell–Boltzmann velocities, and tracked until ejection or capture. Capture is found to be negligible relative to ejection by orders of magnitude, so no global ejection-probability factor is needed.

Two methodological points deserve emphasis. First, the computed spectra contain two components: a low-energy tail from weak distant scatterings whose normalization diverges as the simulation boundary grows, and a physically well-defined high-energy tail from hard encounters within the binary's intrinsic extent, which converges against boundary-size tests. Only the latter is used. Second, the joint probability in angle and energy is factorized, and the angular distribution approximated as isotropic. The appendices justify these choices quantitatively: edge-on ejection probabilities of $2200\ \rm km/s$7 versus face-on values of $2200\ \rm km/s$8 bracket the isotropic value $2200\ \rm km/s$9, and the anisotropy is expected to average out over a population of randomly oriented binaries. These remain approximations rather than exact results, and a fully differential treatment would require substantially more computation.

Why double black holes dominate

Comparing simulated ejection spectra across observed and representative binaries — main-sequence pairs such as $^\star$0 Lyr, ultracompact white dwarf systems like AM CVn and HM Cnc, neutron star binaries like J0737$^\star$13039, and stellar-mass black hole binaries — the paper establishes a clear hierarchy. Systems containing extended objects are limited by tidal disruption radii to wide separations and shallow potentials; white dwarf and neutron star binaries achieve hard spectra but at suppressed rates due to their small masses and deflection cross-sections. Double black holes uniquely combine large total mass ($^\star$2 adopted here), allowing wide orbits without sacrificing potential depth, with extreme compactness permitting close encounters and efficient large-angle deflection. Short-period BBHs with $^\star$3 are identified as the most effective galactic sources.

Galactic populations and fluxes

Three source populations are treated using synthetic catalogues built on FIRE cosmological simulations combined with binary population synthesis:

Population	Key parameters	Maximum ejecta speed
Local BBHs ($^\star$4)	$^\star$5, ~36% in relevant period bins	$^\star$6
Bulge BBHs ($^\star$7 around GC)	$^\star$8 binaries, ~21% contributing	comparable to local
Sgr.~A$^\star$9 + BH companion	$\sim 8300\ \rm km/s$0 benchmark, $\sim 8300\ \rm km/s$1 rescaling verified	$\sim 8300\ \rm km/s$2

For the bulge, an NFW profile with slopes $\sim 8300\ \rm km/s$3 to $\sim 8300\ \rm km/s$4 brackets the halo-contraction uncertainty; the resulting spread propagates directly into the predicted fluxes. A steady-state assumption is validated by comparing the $\sim 8300\ \rm km/s$5 propagation time from the Galactic Center against the $\sim 8300\ \rm km/s$6 gravitational-radiation merger timescale of the shortest-period binaries considered.

The nuclear star cluster analysis is deliberately restricted to order-of-magnitude estimates. Double BBHs orbiting Sgr.~A$\sim 8300\ \rm km/s$7 contribute negligibly because bound DM cannot be unbound except at ultrashort periods, while transiting halo DM requires improbably close encounters at the $\sim 8300\ \rm km/s$8 infall speeds. The dominant candidate system is instead Sgr.~A$\sim 8300\ \rm km/s$9 itself paired with a stellar-mass black hole, treated via a numerically verified $0.5$0 scaling law for the deflection cross-section. Even the single benchmark system produces a velocity distribution extending to $0.5$1 — below the theoretical maximum of $0.5$2 for those orbital parameters, which would require fine-tuned initial conditions. The paper identifies this region, combining the Galaxy's highest DM densities, $0.5$3 BBHs within $0.5$4, and the deepest potential well, as likely the dominant source of binary-boosted DM overall, but leaves its full characterization to future work.

Direct detection sensitivity

Using published low-energy ionization searches from LZ and PandaX-4T (thresholds near $0.5$5), the authors reconstruct SHM-based limits to within factors of $0.5$6–$0.5$7 and then recompute sensitivity with the boosted distributions. For elastic spin-independent scattering, bulge binaries extend the mass reach of LZ to $0.5$8 and PandaX-4T to $0.5$9 at cross-sections below the $1\ \rm GeV$0 overburden threshold. Lower-threshold experiments retain better cross-section sensitivity, but the result demonstrates that existing tonne-scale detectors gain sub-GeV reach through a channel requiring no new particle physics assumptions. Including the nuclear star cluster contribution could push PandaX-4T and possibly DarkSide-50 beyond the lowest masses currently accessed only by CRESST-III via nuclear recoils.

For spin-dependent neutron-only scattering at PandaX-4T, the boosted flux extends the mass reach to $1\ \rm GeV$1 depending on the assumed halo slope; local binaries are too weak to contribute below overburden limits, and proton-only scattering yields no enhancement below $1\ \rm GeV$2.

The most distinctive application is inelastic scattering. Under the SHM, endothermic up-scattering in xenon is kinematically capped at splittings of a few hundred keV, since $1\ \rm GeV$3 at $1\ \rm GeV$4. Binary-boosted velocities raise this ceiling to $1\ \rm GeV$5–$1\ \rm GeV$6 for bulge binaries at TeV DM masses, and potentially to MeV-scale splittings once the nuclear star cluster is included. Crucially, unlike most boosting mechanisms whose efficiency falls with DM mass, the gravitational boost persists at heavy masses, making this the unique feature of the channel.

The paper also sketches extensions to neutrino detectors — viable only in the $1\ \rm GeV$7 regime where recoil energies reach $1\ \rm GeV$8 on carbon — and to mineral-based paleodetectors, while noting that DM–electron scattering in the sub-MeV regime is already tightly constrained by stellar cooling and cosmology, limiting the value of extending that channel.

Limitations and open questions

Several caveats bear directly on the quantitative predictions. The binary population inputs come from a single conservative catalogue; alternative syntheses such as StarTrack predict considerably larger galactic BBH abundances and hence stronger fluxes, so the results should be read as lower bounds on the effect. Eccentricity, mass ratio, barycenter motion, and angular anisotropy are each shown to induce only factor-of-order-unity changes individually, but their combined impact on a full population has not been exhaustively scanned. The nuclear star cluster — argued to be the dominant source — is treated only through a single idealized benchmark system, leaving its integrated flux undetermined. DarkSide-50's argon response, Migdal-effect and bremsstrahlung channels, capture dynamics, and back-reaction of DM on binary evolution are all deferred. Whether neutrino observatories attain novel sensitivity awaits the same complete nuclear-star-cluster calculation.

Conclusion

The paper establishes gravitational scattering in compact binaries as a robust, model-independent accelerator of galactic DM, with double black hole binaries producing ejecta up to $1\ \rm GeV$9 and Sgr.~A$m_\chi \ll M_2 \ll M_1$0 companions reaching $m_\chi \ll M_2 \ll M_1$1. Applied to LZ and PandaX-4T, the boosted flux extends mass sensitivity into the sub-GeV regime for spin-independent and spin-dependent scattering and lifts the inelastic splitting ceiling to $m_\chi \ll M_2 \ll M_1$2 at TeV masses. The principal open question is the magnitude of the nuclear star cluster contribution, which the authors' own arguments suggest could dominate both the flux and the energy reach of the entire mechanism.