Effects of nonstandard halo velocity distributions

Determine how deviations from the Standard Halo Model affect the predicted recoil spectrum, annual modulation, and seasonal behavior of inelastic dark matter signals supplied by the high-speed tail of the Galactic dark-matter velocity distribution near the escape speed.

Background

The paper interprets the LUX-ZEPLIN high-energy nuclear recoil as potentially arising from endothermic inelastic dark matter with a mass splitting close to the kinematic limit imposed by the halo escape speed. In this regime, the signal is generated by dark matter particles in the extreme high-speed tail of the velocity distribution, producing a large annual modulation and potentially a rate that vanishes during part of the year.

All quantitative results are derived under the isothermal Standard Halo Model. Because the relevant high-speed tail is particularly sensitive to the actual Galactic velocity distribution, the paper explicitly identifies determining how departures from that model modify the predicted signal as unresolved future work.

References

The signal is supplied by the tail of the velocity distribution nearest the escape speed, the part about which the model is most uncertain, so how deviations from it would change these results is a question we leave for future work.

— Seasonal dark matter from the LUX-ZEPLIN high-energy event  (2609.04181 - McCabe, 3 Sep 2026) in Summary and outlook

The main unknown, as we elaborate upon below, is the high-velocity tail of the DM velocity distribution in the Milky Way.

— Confronting the Higgsino Interpretation of the LZ Event with the High-Energy Sideband  (2609.04175 - Rodd et al., 3 Sep 2026) in Section 1, introductory discussion of the higgsino interpretation

We fitted the velocity distribution with an isotropic Maxwellian in this study. However, in realistic galactic discs, the velocity dispersions may differ among the three spatial components owing to the structure and dynamical evolution of the galaxy . A component-wise analysis of such an anisotropic velocity distribution could change the impact on direct detection experiments. In that case, the disc-extraction method proposed in this study, which suppresses the halo contribution and preferentially selects DM associated with the disc region, should be well suited to evaluating the disc-specific kinematics and velocity anisotropy. We leave this for future work.

— Disc-Based Estimation of the Local Dark Matter Density and Velocity Distribution in IllustrisTNG50  (2609.02185 - Chujo et al., 2 Sep 2026) in Section Conclusion, following the discussion of the isotropic Maxwellian fit