The high-velocity dark matter halo of the Milky Way in light of the LZ 248 keV event
Abstract
The interpretation of high-energy nuclear recoil events in direct-detection experiments can depend sensitively on the poorly understood high-speed tail of the Galactic dark matter velocity distribution. This fact has been brought into sharper focus recently by an anomalous nuclear-recoil event at ER≈248 keV observed by the LZ experiment. If interpreted as caused by a dark-matter-induced recoil, kinematic constraints on many models imply this particle would have to emerge from the high-speed tail. Here, we re-assess this astrophysical uncertainty using Milky Way analogues from the IllustrisTNG and FIRE simulations. We find that the maximum laboratory-frame DM speed on June 16, when the event was observed, is 792.7+82.9−95.6 km/s (median and 95\% containment across simulations), with a median very close to the fiducial SHM value. Modelling the tail of the velocity distribution in three dimensions is complicated by the fact that these simulated dark matter halos are anisotropic and are generically seen to spin in the same direction as the baryonic disk. However, we find that this does not reduce the maximum laboratory-frame DM speed because the approximately Gaussian azimuthal velocity distribution is negatively skewed by this co-rotation, rather than shifted by it. We illustrate these results in the context of an inelastic dark matter interpretation of the LZ event.
Related
- Higgsino dark matter in the Starobinsky supergravity with the MSSM in light of the LUX-ZEPLIN event
- Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event
- Resurrecting Electroweak Dark Matter via Type-II Seesaw in light of recent LZ Event
- Sub-TeV Singlino Dark Matter in light from Sagittarius A∗ and LUX-ZEPLIN Nuclear-Recoil Event
- GAIA meets LZ: a high velocity tail-tale sign for dark matter
BibTeX
@misc{ohare2026highvelocity,
title = {{The high-velocity dark matter halo of the Milky Way in light of the LZ 248 keV event}},
author = {Ciaran A. J. O'Hare},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.21444},
eprint = {2609.21444},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.21444},
}
RIS
TY - UNPB
TI - The high-velocity dark matter halo of the Milky Way in light of the LZ 248 keV event
AU - Ciaran A. J. O'Hare
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.21444
UR - https://arxiv.org/abs/2609.21444
AB - The interpretation of high-energy nuclear recoil events in direct-detection experiments can depend sensitively on the poorly understood high-speed tail of the Galactic dark matter velocity distribution. This fact has been brought into sharper focus recently by an anomalous nuclear-recoil event at E_R≈248 keV observed by the LZ experiment. If interpreted as caused by a dark-matter-induced recoil, kinematic constraints on many models imply this particle would have to emerge from the high-speed tail. Here, we re-assess this astrophysical uncertainty using Milky Way analogues from the IllustrisTNG and FIRE simulations. We find that the maximum laboratory-frame DM speed on June 16, when the event was observed, is 792.7^+82.9_−95.6 km/s (median and 95\% containment across simulations), with a median very close to the fiducial SHM value. Modelling the tail of the velocity distribution in three dimensions is complicated by the fact that these simulated dark matter halos are anisotropic and are generically seen to spin in the same direction as the baryonic disk. However, we find that this does not reduce the maximum laboratory-frame DM speed because the approximately Gaussian azimuthal velocity distribution is negatively skewed by this co-rotation, rather than shifted by it. We illustrate these results in the context of an inelastic dark matter interpretation of the LZ event.
ER -