Late decaying 2-component dark matter scenario as an explanation of the AMS-02 positron excess

TL;DR

This paper proposes a late-decaying two-component dark matter model where the decay of a heavier species produces lighter dark matter particles, explaining the AMS-02 positron excess without conflicting with CMB constraints.

Contribution

It introduces a novel late-decaying two-component dark matter framework that accounts for positron excess and satisfies cosmological constraints, with a simple non-thermal production mechanism.

Findings

Explains AMS-02 positron excess with enhanced dark matter annihilation cross-section.

Avoids CMB constraints on energy deposition during cosmic dark ages.

Reproduces observed dark matter relic density with simple s-wave annihilation.

Abstract

The long standing anomaly in the positron flux as measured by the PAMELA and AMS-02 experiments could potentially be explained by dark matter (DM) annihilations. This scenario typically requires a large "boost factor" to be consistent with a thermal relic dark matter candidate produced via freeze-out. However, such an explanation is disfavored by constraints from CMB observations on energy deposition during the epoch of recombination. We discuss a scenario called late-decaying two-component dark matter (LD2DM), where the entire DM consists of two semi-degenerate species. Within this framework, the heavier species is produced as a thermal relic in the early universe and decays to the lighter species over cosmological timescales. Consequently, the lighter species becomes the DM which populates the universe today. We show that annihilation of the lighter DM species with an enhanced cross-section, produced via such a non-thermal mechanism, can explain the observed AMS-02 positron flux while avoiding CMB constraints. The observed DM relic density can be correctly reproduced as well with simple s-wave annihilation cross-sections. We demonstrate that the scenario is safe from CMB constraints on late-time energy depositions during the cosmic "dark ages". Interestingly, structure formation constraints force us to consider small mass splittings between the two dark matter species. We explore possible cosmological and particle physics signatures in a toy model that realizes this scenario.