Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN
Abstract
The single 248-keV nuclear-recoil candidate reported by LZ is reproduced by dark matter that downscatters, with the relic in the excited state of a pseudo-Dirac pair and a splitting δ\simeq-350\keV. Every interpretation so far advanced is endothermic and places the splitting within a few percent of the xenon kinematic ceiling, so that the predicted count changes from zero to 143 times its central value as the halo escape speed runs from 500 to 600 km/s. The downscattering branch has no threshold speed; the same variation changes its count by 0.3 %. The ratio of counts below 70 keV to those above is 0.011, so the null standard search follows from the kinematics. A weak-strength coupling overproduces the event by 8.9\times10^{5}, so the mediator is a dark-sector one, and longevity of the relic excited state requires |δ|<2m_e. Solving the coupled Boltzmann system removes the one free normalization. Rotating to the eigenchannels of the off-diagonal potential gives σ_0=(π/k^2)\sin^2(δ_+-δ_-), whose phase difference is 0.03 to 1.3 rad, so the Born rate exceeds the s-wave unitarity bound where it is used. One event then requires σ_p\simeq2\times10^{-42} cm^2 and a kinetic mixing ε\simeq1\times10^{-6} at m_{A'}=1\GeV, with beam-dump coverage below 0.6\GeV pushing the mediator above that value. The ground state upscatters endothermically through the same operator, which excludes |δ|\lesssim300 keV and makes the published endothermic reading the small-splitting limit of one model. The decisive test is argon, whose first form-factor zero lies at 695 keV against 94.6 keV for xenon; argon yields 4 to 6 events per tonne-year near 250 keV, where the endothermic reading is forbidden at any exposure.
Related
- Higgsino Dark Matter Interpretation of the LUX-ZEPLIN 248 keV Nuclear-Recoil Event
- Endothermic dark matter with a light dark photon and the LUX-ZEPLIN high-energy nuclear-recoil candidate
- Sub-TeV Singlino Dark Matter in light from Sagittarius A∗ and LUX-ZEPLIN Nuclear-Recoil Event
- Higgsino dark matter compatible with the LUX-ZEPLIN high-energy nuclear-recoil event and IceCube constraints
- Search for dark matter particle interactions in an extended nuclear recoil energy window with the LUX-ZEPLIN (LZ) experiment
BibTeX
@misc{baer2026exothermic,
title = {{Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN}},
author = {Howard Baer and Vernon Barger},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.06153},
eprint = {2609.06153},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.06153},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN
AU - Howard Baer
AU - Vernon Barger
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.06153
UR - https://arxiv.org/abs/2609.06153
AB - The single 248-keV nuclear-recoil candidate reported by LZ is reproduced by dark matter that downscatters, with the relic in the excited state of a pseudo-Dirac pair and a splitting δ\simeq-350\keV. Every interpretation so far advanced is endothermic and places the splitting within a few percent of the xenon kinematic ceiling, so that the predicted count changes from zero to 143 times its central value as the halo escape speed runs from 500 to 600 km/s. The downscattering branch has no threshold speed; the same variation changes its count by 0.3 %. The ratio of counts below 70 keV to those above is 0.011, so the null standard search follows from the kinematics. A weak-strength coupling overproduces the event by 8.9\times10^{5}, so the mediator is a dark-sector one, and longevity of the relic excited state requires |δ|<2m_e. Solving the coupled Boltzmann system removes the one free normalization. Rotating to the eigenchannels of the off-diagonal potential gives σ_0=(π/k^2)\sin^2(δ_+-δ_-), whose phase difference is 0.03 to 1.3 rad, so the Born rate exceeds the s-wave unitarity bound where it is used. One event then requires σ_p\simeq2\times10^{-42} cm^2 and a kinetic mixing ε\simeq1\times10^{-6} at m_{A'}=1\GeV, with beam-dump coverage below 0.6\GeV pushing the mediator above that value. The ground state upscatters endothermically through the same operator, which excludes |δ|\lesssim300 keV and makes the published endothermic reading the small-splitting limit of one model. The decisive test is argon, whose first form-factor zero lies at 695 keV against 94.6 keV for xenon; argon yields 4 to 6 events per tonne-year near 250 keV, where the endothermic reading is forbidden at any exposure.
ER -