Low-Temperature Enhancement of Semi-annihilation and the AMS-02 Positron Anomaly

TL;DR

This paper investigates how low-temperature resonance effects can enhance semi-annihilation of dark matter, potentially explaining the AMS-02 positron excess while satisfying relic density and observational constraints.

Contribution

It introduces a model with late-time semi-annihilation enhancement via s-channel resonance, providing a new mechanism for dark matter signals compatible with observations.

Findings

Semi-annihilation cross section can be boosted by up to five orders of magnitude.

The model fits AMS-02 positron data without conflicting with relic density.

Constraints from CMB and gamma-ray observations limit parameter space.

Abstract

Semi-annihilation is a generic feature of particle dark matter that is most easily probed by cosmic ray experiments. We explore models where the semi-annihilation cross section is enhanced at late times and low temperatures by the presence of an s-channel resonance near threshold. The relic density is then sensitive to the evolution of the dark matter temperature, and we compute expressions for the associated Boltzmann equation valid in general semi-annihilating models. At late times, a self-heating effect warms the dark matter, allowing number-changing processes to remain effective long after kinetic decoupling of the dark and visible sectors. This allows the semi-annihilation signal today to be enhanced by up to five orders of magnitude over the thermal relic cross section. As a case study, we apply this to a dark matter explanation of the positron excess seen by AMS-02. We see that unlike annihilating dark matter, our model has no difficulty fitting the data while also giving the correct relic density. However, constraints from the CMB and $\gamma$-rays from the galactic centre do restrict the preferred regions of parameter space.