Inelastic Kinetic Decoupling in WIMP Leptogenesis: A Minimal Z4 Model
Abstract
We study a minimal Z4-symmetric extension of the Standard Model in which a singlet Dirac fermion χ constitutes dark matter, whose annihilations generate a lepton asymmetry through WIMP leptogenesis. Kinetic equilibrium is maintained mainly by the inelastic conversion χL↔χ̅Ψ. Compared with the near elastic regime, a sizable mass gap suppresses the conversion rate more rapidly as the Universe cools. This naturally drives kinetic decoupling to earlier times, even away from resonance. Starting from the full inelastic collision operator, we derive an effective momentum transfer rate for kinetic decoupling. We then solve the coupled Boltzmann equations for the dark matter abundance, its temperature Tχ, and the B−L asymmetry. Early kinetic decoupling can significantly modify the relic abundance, although near resonance this modification can be reduced by annihilation-induced heating. The B−L asymmetry can be affected when asymmetry production overlaps with a substantial departure from kinetic equilibrium. In our scan, the largest deviations occur mainly for xlep≳ xcd. After imposing our constraints, asymmetry production typically ends before chemical decoupling, leaving the final asymmetry essentially unchanged. For indirect detection, we generate the photon spectrum with Pythia and evaluate the Fermi-LAT dwarf spheroidal likelihood.
Related
- Detection prospects for heavy WIMP dark matter around M31* in microwave band
- Dynamical Tsallis WIMP Freeze-Out and Residual Memory Channels in the Radiation Sector
- Dwarf Galaxy Reticulum II's MeerKAT L-Band Observation: Constraining WIMP Dark Matter
- Decay-Assisted Leptogenesis during Preheating
- Leptogenesis with sub-electroweak-scale reheating temperature
BibTeX
@misc{liu2026inelastic,
title = {{Inelastic Kinetic Decoupling in WIMP Leptogenesis: A Minimal $Z_4$ Model}},
author = {An Liu and Hong-Hao Zhang},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.38531},
eprint = {2610.38531},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.38531},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Inelastic Kinetic Decoupling in WIMP Leptogenesis: A Minimal Z_4 Model
AU - An Liu
AU - Hong-Hao Zhang
PY - 2026
DA - 2026/10/28
PB - arXiv
DO - 10.48550/arxiv.2610.38531
UR - https://arxiv.org/abs/2610.38531
AB - We study a minimal Z_4-symmetric extension of the Standard Model in which a singlet Dirac fermion χ constitutes dark matter, whose annihilations generate a lepton asymmetry through WIMP leptogenesis. Kinetic equilibrium is maintained mainly by the inelastic conversion χL↔χ̅Ψ. Compared with the near elastic regime, a sizable mass gap suppresses the conversion rate more rapidly as the Universe cools. This naturally drives kinetic decoupling to earlier times, even away from resonance. Starting from the full inelastic collision operator, we derive an effective momentum transfer rate for kinetic decoupling. We then solve the coupled Boltzmann equations for the dark matter abundance, its temperature T_χ, and the B−L asymmetry. Early kinetic decoupling can significantly modify the relic abundance, although near resonance this modification can be reduced by annihilation-induced heating. The B−L asymmetry can be affected when asymmetry production overlaps with a substantial departure from kinetic equilibrium. In our scan, the largest deviations occur mainly for x_lep≳ x_cd. After imposing our constraints, asymmetry production typically ends before chemical decoupling, leaving the final asymmetry essentially unchanged. For indirect detection, we generate the photon spectrum with Pythia and evaluate the Fermi-LAT dwarf spheroidal likelihood.
ER -