Boosted or Inelastic? Discriminating Interpretations of the LZ 248 keV Event
Abstract
The LUX-ZEPLIN experiment has reported a single nuclear recoil candidate at ER = 248 ± 23(stat) ± 23(sys)~keV, disfavouring the background-only hypothesis at a global significance of 2.6σ. The difficulty such an event poses is not the recoil energy itself but the absence of any accompanying excess at low energy as elastic scattering of halo dark matter yields a monotonically falling spectrum, and supplying the required momentum transfer q ≃ 246~MeV already demands mχ≳ 79~GeV. We confront the event with the two kinematically distinct mechanisms that evade this limitation, endothermic inelastic dark matter, in which a mass splitting δ∼ 𝒪(100)~keV forbids low-energy recoils and boosted dark matter, in which a light relativistic flux supplies the momentum, treating both with the same model-independent scalar--scalar, pseudoscalar--scalar and pseudoscalar--pseudoscalar effective operators. The two scenarios prove spectrally distinguishable. The inelastic spectra sit near the observed energy for every operator, placing only $6−−17\%$ of events below 150~keV, whereas the boosted spectra depend critically on the operator: the scalar and pseudoscalar--scalar interactions place 99% and 92% of their events below 150~keV, while the pseudoscalar--pseudoscalar interaction places 75% above it. Momentum dependence is thus essential to the boosted interpretation, but in the inelastic case it trades against the splitting, the preferred δ decreasing monotonically from 𝒪ss to 𝒪ps to 𝒪pp. Because the scenarios differ across the whole high-energy window, a handful of additional events would separate them, placing the question within reach of the full LZ exposure.
Related
- Inelastic Singlet-Doublet Fermion Dark Matter in light of the 248 keV LZ event
- LZ nuclear recoil event from inelastic singlet-doublet scalar dark matter
- Inelastic Dark Photon Dark Matter for the LUX-ZEPLIN High-Recoil Event and the Galactic Halo Gamma-Ray Excess
- Hadrophilic inelastic freeze-in dark matter in q1−q2 gauge extension and the high energy LZ event
- Inelastic Self-interacting Dark Matter and LUX-ZEPLIN 248 keV Event in a Dirac Modular Inverse Seesaw
BibTeX
@misc{mahapatra2026boosted,
title = {{Boosted or Inelastic? Discriminating Interpretations of the LZ 248 keV Event}},
author = {Satyabrata Mahapatra and Partha Kumar Paul},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.14799},
eprint = {2609.14799},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.14799},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Boosted or Inelastic? Discriminating Interpretations of the LZ 248 keV Event
AU - Satyabrata Mahapatra
AU - Partha Kumar Paul
PY - 2026
DA - 2026/09/13
PB - arXiv
DO - 10.48550/arxiv.2609.14799
UR - https://arxiv.org/abs/2609.14799
AB - The LUX-ZEPLIN experiment has reported a single nuclear recoil candidate at E_R = 248 ± 23(stat) ± 23(sys)~keV, disfavouring the background-only hypothesis at a global significance of 2.6σ. The difficulty such an event poses is not the recoil energy itself but the absence of any accompanying excess at low energy as elastic scattering of halo dark matter yields a monotonically falling spectrum, and supplying the required momentum transfer q ≃ 246~MeV already demands m_χ≳ 79~GeV. We confront the event with the two kinematically distinct mechanisms that evade this limitation, endothermic inelastic dark matter, in which a mass splitting δ∼ 𝒪(100)~keV forbids low-energy recoils and boosted dark matter, in which a light relativistic flux supplies the momentum, treating both with the same model-independent scalar--scalar, pseudoscalar--scalar and pseudoscalar--pseudoscalar effective operators. The two scenarios prove spectrally distinguishable. The inelastic spectra sit near the observed energy for every operator, placing only $6−−17\%$ of events below 150~keV, whereas the boosted spectra depend critically on the operator: the scalar and pseudoscalar--scalar interactions place 99% and 92% of their events below 150~keV, while the pseudoscalar--pseudoscalar interaction places 75% above it. Momentum dependence is thus essential to the boosted interpretation, but in the inelastic case it trades against the splitting, the preferred δ decreasing monotonically from 𝒪_ss to 𝒪_ps to 𝒪_pp. Because the scenarios differ across the whole high-energy window, a handful of additional events would separate them, placing the question within reach of the full LZ exposure.
ER -