Inelastic Kinetic Decoupling in WIMP Leptogenesis: A Minimal $Z_4$ Model

Abstract

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 $χL\leftrightarrow\barχΨ$. 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 $x_{\rm lep}\gtrsim x_{\rm cd}$. 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.