Cosmic-Ray Signatures of Annihilating and Semi-Annihilating Dark Matter via One-Step Cascades

Abstract

We present a framework in which three classes of dark matter number-changing processes can affect both the relic abundance via thermal freeze-out in the early universe and the generation of indirect cosmic-ray signals today. These processes are: (i) direct annihilations into Standard Model final states; (ii) annihilations into metastable on-shell mediators that subsequently decay into Standard Model particles; (iii) semi-annihilation processes featuring a dark matter particle in the final state, accompanied by a metastable mediator. A central element of our analysis is the systematic inclusion of semi-annihilation alongside the more commonly considered channels. This setup is largely model-independent, as we only assume the presence of one or more of these processes with unsuppressed $s$-wave contributions. We analyze representative benchmarks for the dominant decay modes of the mediator and show how the resulting injection spectra for $\gamma$ rays, neutrinos, and cosmic-ray antimatter vary with the relative importance of the three classes of processes. As an application, we evaluate the observable $\gamma$-ray fluxes from dwarf spheroidal galaxies in the GeV-TeV window. Finally, we provide explicit model realizations in which multiple processes coexist, and discuss how their interplay shapes indirect detection signatures. Our results provide a consistent connection between early-universe dynamics and present-day observables, revealing distinctive features that arise when multiple dark matter processes contribute simultaneously.