Exothermic dark-photon dark matter and the LZ high-recoil event: finite-reheating constraints
Abstract
An excited dark-matter state can produce a high-energy nuclear recoil by releasing its mass splitting in a collision. Motivated by the $248$~keV LUX--ZEPLIN candidate, we study freeze-in of a $45$~GeV pseudo-Dirac particle with a $1$~MeV splitting and a $1$~GeV dark photon during non-instantaneous reheating. We choose a recoil normalization compatible with the published INTEGRAL/SPI three-photon decay limit. It predicts $0.43$ events in the LZ window and retains the reference recoil shape. Including both dark-photon and Z exchange, we calculate production and entropy dilution for backgrounds with T∝ a−α and ρϕ∝ a−3(1+ω). For Tmax=mχ, the observed abundance requires Trh=1.32~GeV for matter-like constant-width reheating, $3.09$~GeV for quartic oscillations with scalar decay, and $0.588$~GeV for quartic oscillations with constant width, compared with $4.10$~GeV for instantaneous reheating. These finite-reheating solutions are insensitive to increasing Tmax above mχ. For Tmax≥ mχ, adiabatic kination has no solution with Trh>5.96~MeV, the BBN-motivated cutoff adopted for our scans. At αD=10−10, the estimated thermal-mediator source is below 0.2% of the Standard Model source, and self-conversion does not appreciably deplete the excited population. The calculation relates an exothermic recoil benchmark to the production and dilution history before radiation domination.
Related
- Decaying Dark Matter for the Exothermic Recoil at LZ
- Heavy Higgsino Dark Matter With Low Reheating in View of the LZ High-Recoil Event
- Exothermic dark matter and the 248 keV nuclear recoil in LUX-ZEPLIN
- Exothermic and Endothermic Inelastic Dark Matter Interpretations at LZ: Sideband Constraints and Future Prospects
- Dark photon portal dark matter with low-temperature reheating
BibTeX
@misc{deka2026exothermic,
title = {{Exothermic dark-photon dark matter and the LZ high-recoil event: finite-reheating constraints}},
author = {Kuldeep Deka},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.18974},
eprint = {2610.18974},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.18974},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Exothermic dark-photon dark matter and the LZ high-recoil event: finite-reheating constraints
AU - Kuldeep Deka
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.18974
UR - https://arxiv.org/abs/2610.18974
AB - An excited dark-matter state can produce a high-energy nuclear recoil by releasing its mass splitting in a collision. Motivated by the $248$~keV LUX--ZEPLIN candidate, we study freeze-in of a $45$~GeV pseudo-Dirac particle with a $1$~MeV splitting and a $1$~GeV dark photon during non-instantaneous reheating. We choose a recoil normalization compatible with the published INTEGRAL/SPI three-photon decay limit. It predicts $0.43$ events in the LZ window and retains the reference recoil shape. Including both dark-photon and Z exchange, we calculate production and entropy dilution for backgrounds with T∝ a^−α and ρ_ϕ∝ a^−3(1+ω). For Tmax=m_χ, the observed abundance requires Trh=1.32~GeV for matter-like constant-width reheating, $3.09$~GeV for quartic oscillations with scalar decay, and $0.588$~GeV for quartic oscillations with constant width, compared with $4.10$~GeV for instantaneous reheating. These finite-reheating solutions are insensitive to increasing Tmax above m_χ. For Tmax≥ m_χ, adiabatic kination has no solution with Trh>5.96~MeV, the BBN-motivated cutoff adopted for our scans. At α_D=10^−10, the estimated thermal-mediator source is below 0.2% of the Standard Model source, and self-conversion does not appreciably deplete the excited population. The calculation relates an exothermic recoil benchmark to the production and dilution history before radiation domination.
ER -