Thermal contact and direct detection in a broken Z4 Majorana dark matter model
Abstract
We consider a broken Z4 Majorana dark matter model in which the fermion mass is generated by a real singlet scalar. For small scalar mixing, freeze-out is dominated by χχ→ h2h2, whereas momentum exchange with the Standard Model bath becomes inefficient. In this region, the usual assumption Tχ=T is not always adequate. We therefore evolve the dark matter abundance and kinetic temperature simultaneously. Although Tχ is still close to the bath temperature around chemical freeze-out, the later departure from kinetic equilibrium reduces the residual p-wave annihilation rate and leaves a larger relic abundance. In the sub-TeV region considered here, the difference between the two-moment and single-temperature calculations reaches 37.1% at fixed parameters. Refitting the Yukawa coupling to reproduce Ωχh2=0.12 increases the spin-independent scattering cross section by up to 18.6%. A momentum-resolved Fokker--Planck calculation gives a similar shift, 17.7%, at a representative point. The LZ upper limit on the scalar mixing changes by less than 0.3%, since kinetic equilibrium is already well maintained in that part of parameter space. The lower mixing boundary obtained from the comparison of the two relic calculations therefore indicates where the common-temperature approximation loses accuracy; it is not an experimental lower bound. These results apply to the specified low-reheating broken-phase cosmology and to the region where threshold and strong nonrelativistic-potential effects remain small.
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BibTeX
@misc{qi2026thermal,
title = {{Thermal contact and direct detection in a broken $Z_4$ Majorana dark matter model}},
author = {XinXin Qi and Hao Sun},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.11936},
eprint = {2606.11936},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.11936},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Thermal contact and direct detection in a broken Z_4 Majorana dark matter model
AU - XinXin Qi
AU - Hao Sun
PY - 2026
DA - 2026/10/07
PB - arXiv
DO - 10.48550/arxiv.2606.11936
UR - https://arxiv.org/abs/2606.11936
AB - We consider a broken Z_4 Majorana dark matter model in which the fermion mass is generated by a real singlet scalar. For small scalar mixing, freeze-out is dominated by χχ→ h_2h_2, whereas momentum exchange with the Standard Model bath becomes inefficient. In this region, the usual assumption T_χ=T is not always adequate. We therefore evolve the dark matter abundance and kinetic temperature simultaneously. Although T_χ is still close to the bath temperature around chemical freeze-out, the later departure from kinetic equilibrium reduces the residual p-wave annihilation rate and leaves a larger relic abundance. In the sub-TeV region considered here, the difference between the two-moment and single-temperature calculations reaches 37.1% at fixed parameters. Refitting the Yukawa coupling to reproduce Ω_χh^2=0.12 increases the spin-independent scattering cross section by up to 18.6%. A momentum-resolved Fokker--Planck calculation gives a similar shift, 17.7%, at a representative point. The LZ upper limit on the scalar mixing changes by less than 0.3%, since kinetic equilibrium is already well maintained in that part of parameter space. The lower mixing boundary obtained from the comparison of the two relic calculations therefore indicates where the common-temperature approximation loses accuracy; it is not an experimental lower bound. These results apply to the specified low-reheating broken-phase cosmology and to the region where threshold and strong nonrelativistic-potential effects remain small.
ER -