Decaying Dark Matter for the Exothermic Recoil at LZ
Abstract
The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at 248±23 (stat)±23 (sys) keV. We suggest that it comes from a GeV-scale dark state χ1 that downscatters exothermically, χ1A→χ2A, and is continuously replenished in the halo by the slow decay χ0→χ1χ1 of a long-lived parent. Because the recoil energy is supplied by the mass splitting rather than by the halo velocity, the event is a narrow line with no low-energy tail. Kinematics requires mχ1≳247 MeV and fixes the splitting for each mχ1. The same line should appear at 700−810 keV in argon for mχ1=0.3−10 GeV. Normalizing to one LZ event also fixes the local rate of the radiative decay χ1→χ2γγ, up to the mediator's photon coupling. A rough γ-ray estimate then requires this coupling to be ≲10−2 of a proton-loop reference at mχ1=10 GeV, but ≲10−9 at 0.3 GeV, which favours daughters near 10 GeV.
Related
- Exothermic dark-photon dark matter and the LZ high-recoil event: finite-reheating constraints
- Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN
- Exothermic Dark Matter with a Dark Higgs: The LZ 248 keV Event and the Galactic Center Excess
- Exothermic Dark Matter at LZ
- Exothermic nuclear recoils from thermal quasi-pNGB dark matter
BibTeX
@misc{jho2026decaying,
title = {{Decaying Dark Matter for the Exothermic Recoil at LZ}},
author = {Yongsoo Jho and Sanghwan Kim and Seong Chan Park},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.37561},
eprint = {2609.37561},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.37561},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Decaying Dark Matter for the Exothermic Recoil at LZ
AU - Yongsoo Jho
AU - Sanghwan Kim
AU - Seong Chan Park
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2609.37561
UR - https://arxiv.org/abs/2609.37561
AB - The LUX-ZEPLIN (LZ) experiment has reported a single nuclear-recoil candidate at 248±23 (stat)±23 (sys) keV. We suggest that it comes from a GeV-scale dark state χ_1 that downscatters exothermically, χ_1A→χ_2A, and is continuously replenished in the halo by the slow decay χ_0→χ_1χ_1 of a long-lived parent. Because the recoil energy is supplied by the mass splitting rather than by the halo velocity, the event is a narrow line with no low-energy tail. Kinematics requires m_χ_1≳247 MeV and fixes the splitting for each m_χ_1. The same line should appear at 700−810 keV in argon for m_χ_1=0.3−10 GeV. Normalizing to one LZ event also fixes the local rate of the radiative decay χ_1→χ_2γγ, up to the mediator's photon coupling. A rough γ-ray estimate then requires this coupling to be ≲10^−2 of a proton-loop reference at m_χ_1=10 GeV, but ≲10^−9 at 0.3 GeV, which favours daughters near 10 GeV.
ER -