Interpretations of indirect searches for dark matter (DM) require theoretical predictions for the annihilation or decay rates of DM into stable particles of the standard model. These predictions include usually only final states accessible as lowest
order tree-level processes, with electromagnetic bremsstrahlung and the loop-suppressed two gamma-ray line as exceptions. We show that this restriction may lead to severely biased results for DM tailored to produce only leptons in final states and with mass in the TeV range. For such models, unavoidable electroweak bremsstrahlung of Z and W-bosons has a significant influence both on the branching ratio and the spectral shape of the final state particles. We work out the consequences for two situations: Firstly, the idealized case where DM annihilates at tree level with 100% branching ratio into neutrinos. For a given cross section, this leads eventually to minimal yields of photons, electrons, positrons and antiprotons. Secondly, the case where the only allowed two-body final states are electrons. The latter case is typical of models aimed at fitting cosmic ray e^- and e^+ data. We find that the multimessenger signatures of such models can be significantly modified with respect to results presented in the literature.
An upper limit on the total annihilation cross section of dark matter (DM) has recently been derived from the observed atmospheric neutrino background. We show that comparable bounds are obtained for DM masses around the TeV scale by observations of
the diffuse gamma-ray flux by EGRET, because electroweak bremsstrahlung leads to non-negligible electromagnetic branching ratios, even if DM particles only couple to neutrinos at tree level. A better mapping and the partial resolution of the diffuse gamma-ray background into astrophysical sources by the GLAST satellite will improve this bound in the near future.
