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

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.

Dark matter decaying in the late Universe can relieve the H0 tension

“…On the other hand, modifications to the dark matter sector in order to resolve the discrepancy include partially acoustic dark matter models [27], charged dark matter with chiral photons [28], dissipative dark matter models [29], cannibal dark matter [30] and axions [31]. Decaying dark matter models were also considered in combination with solving other problems [32][33][34][35][36][37][38][39]. Finally, models of interacting dark matter-dark energy [40][41][42] as well as modifications of the general relativity theory [43][44][45] were discussed.…”

Cosmic-ray signatures of dark matter from a flavor dependent gauge symmetry model with neutrino mass mechanism

“…The pulsar scenario is supported by the discovery of γ-ray emission around nearby pulsars [72,73] and thus chosen over other discussed explanations of the positron excess such as secondary production in dense clouds around SNRs [74][75][76][77][78][79][80]. While a DM-only explanation of the positron excess is also not ruled out, it requires specific conditions such as decaying dark matter yielding softer spectra than the electron and muon channel annihilation of our DM candidate [81][82][83].…”

Impact of Cosmic-Ray Physics on Dark Matter Indirect Searches

“…On the other hand, many DM interpretations were invoked as well: the tough challenges for model building are in this case the large annihilation cross section required to sustain the measured positron flux at high energy and the strong constraints originating from other channels (including gamma rays, CMB, and antiprotons); we refer [201] for an early review on the topic. The Early Universe bounds [202,203] are particularly difficult to evade, and one has to invoke, for instance, nonthermal mechanisms (see, e.g., [204] for a recent two-component scenario in which the heavier DM species is produced as a thermal relic in the Early Universe and decays to the lighter species over cosmological timescales) or Breit-Wigner enhancement of present-time annihilation (already proposed in 2009 to explain the boost factor required by PAMELA data [205,206] and recently reconsidered, e.g., in [207,208], as a way to evade the stringent CMB constraints).…”