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Numerous observations point towards the existence of an unknown elementary particle with no electromagnetic interactions, a large population of which was presumably produced in the early stages of the history of the Universe. This so-called dark matter has survived until the present day, accounting for the 26% of the present energy budget of the Universe. It remains an open question whether the particles comprising the dark matter are absolutely stable or whether they have a finite but very long lifetime, which is a possibility since there is no known general principle guaranteeing perfect stability. In this paper, we review the observational limits on the lifetime of dark matter particles with mass in the GeV-TeV range using observations of the cosmic fluxes of antimatter, gamma-rays and neutrinos. We also examine some theoretically motivated scenarios that provide decaying dark matter candidates.

The scenario of gravitino dark matter with broken R-parity naturally reconciles three paradigms that, albeit very well motivated separately, seem to be in mutual conflict: supersymmetric dark matter, thermal leptogenesis and standard Big Bang nucleosynthesis. Interestingly enough, the products of the gravitino decay could be observed, opening the possibility of indirect detection of gravitino dark matter. In this paper, we compute the positron and the antiproton fluxes from gravitino decay. We find that a gravitino with a mass of m 3/2 ∼ 150 GeV and a lifetime of τ 3/2 ∼ 10 26 s could simultaneously explain the EGRET anomaly in the extragalactic diffuse gamma ray background and the HEAT excess in the positron fraction. However, the predicted antiproton flux tends to be too large, although the prediction suffers from large uncertainties and might be compatible with present observations for certain choices of propagation parameters.

We investigate different neutrino signals from the decay of dark matter particles to determine the prospects for their detection, and more specifically if any spectral signature can be disentangled from the background in present and future neutrino observatories. If detected, such a signal could bring an independent confirmation of the dark matter interpretation of the dramatic rise in the positron fraction above 10 GeV recently observed by the PAMELA satellite experiment and offer the possibility of distinguishing between astrophysical sources and dark matter decay or annihilation. In combination with other signals, it may also be possible to distinguish among different dark matter decay channels.

A series of experiments measuring high-energy cosmic rays have recently reported strong indications for the existence of an excess of high-energy electrons and positrons. If interpreted in terms of the decay of dark matter particles, the PAMELA measurements of the positron fraction and the Fermi LAT measurements of the total electron-plus-positron flux restrict the possible decaying dark matter scenarios to a few cases. Analyzing different decay channels in a model-independent manner, and adopting a conventional diffusive reacceleration model for the background fluxes of electrons and positrons, we identify some promising scenarios of dark matter decay and calculate the predictions for the diffuse extragalactic gamma-ray flux, including the contributions from inverse Compton scattering with the interstellar radiation field. * Electronic address: alejandro.ibarra@ph.tum.de † Electronic address: david.tran@ph.tum.de ‡ Electronic address: christoph.weniger@desy.de 1 I. INTRODUCTION Different experiments measuring high-energy cosmic rays have over the last months reported a wealth of new results pointing to the existence of an exotic source of electrons and positrons. The PAMELA collaboration reported evidence for a sharp rise of the positron fraction at energies 7 − 100 GeV [1], possibly extending toward even higher energies, compared to the expectations from spallation of primary cosmic rays on the interstellar medium [2]. This result confirmed previous hints about the existence of a positron excess from HEAT [3], CAPRICE [4] and AMS-01 [5]. Almost at the same time, the balloon-borne experiments ATIC [6] and PPB-BETS [7] reported the discovery of a peak in the total electron-pluspositron flux at energies 600 − 700 GeV, while the H.E.S.S. collaboration [8] reported a substantial steepening in the high-energy electron-plus-positron spectrum above 600 GeV compared to lower energies.These results raised a lot of interest in the astrophysics and particle physics communities, leading to many proposals trying to explain this excess. One of the most popular astrophysical interpretations of the positron excess is in terms of the electron-positron pairs produced by the interactions of high-energy photons in the strong magnetic field of pulsars [9,10,11].However, this interpretation requires a rather large fraction of the spin-down power being injected in the form of electron-positron pairs or a rather large rate of gamma-ray pulsar formation. Alternatively, the positrons could be originating from the decay of charged pions, which are in turn produced by the hadronic interactions of high-energy protons accelerated by nearby sources [12].An arguably more exciting explanation of the cosmic-ray positron excess is the possibility that the positrons are produced in the annihilation or the decay of dark matter particles.Should this interpretation be confirmed by future experiments, then the positron excess would constitute the first non-gravitational evidence for the existence of dark matter in our Galaxy. The interpretation of the...