Dark Matter Signals from Cascade Annihilations
  A leading interpretation of the electron/positron excesses seen by PAMELA and
ATIC is dark matter annihilation in the galactic halo. Depending on the
annihilation channel, the electron/positron signal could be accompanied by a
galactic gamma ray or neutrino flux, and the non-detection of such fluxes
constrains the couplings and halo properties of dark matter. In this paper, we
study the interplay of electron data with gamma ray and neutrino constraints in
the context of cascade annihilation models, where dark matter annihilates into
light degrees of freedom which in turn decay into leptons in one or more steps.
Electron and muon cascades give a reasonable fit to the PAMELA and ATIC data.
Compared to direct annihilation, cascade annihilations can soften gamma ray
constraints from final state radiation by an order of magnitude. However, if
dark matter annihilates primarily into muons, the neutrino constraints are
robust regardless of the number of cascade decay steps. We also examine the
electron data and gamma ray/neutrino constraints on the recently proposed
"axion portal" scenario.
