Dark matter indirect signals with long-lived mediators

Dark matter particles could annihilate into light and metastable mediators subsequently decaying far away from where they are produced. In this scenario, the indirect signatures of dark matter are altered with respect to the conventional situation where standard model particles are directly injected where annihilation happens. We revisit the long-lived particle proposal (Rothstein et al., 2009) and devise the tools to explore this new phenomenology. We calculate the effective dark matter distribution resulting from the smearing by mediator propagation. We derive general expressions for the fluxes of mediators and their decay particles. We study how the J-factor, which naturally appears in the calculation of the dark matter induced gamma ray signal, is modified in the presence of mediators. We also derive the anisotropy which the cosmic ray positron flux exhibits in this scenario. We finally comment upon a recent proposal based on long-lived mediators where the effective dark matter density at the Earth is increased such as to explain the cosmic ray positron anomaly. We conclude that this scenario is barely tenable as regards the very dense dark matter spike which it requires at the Galactic center. The associated positron anisotropy is very small and undetectable, except at high energies where it reaches a level of order 10(-4) to 10(-3).